My son came home upset again today. He mentioned some kids at school keep picking on him and I honestly do not know where to start. Has anyone dealt with this before?
IMO, this is one of those situations where listening properly matters more than jumping to solutions. 
When my nephew went through something similar, the first thing we did was just sit with him and let him talk. Children often need to feel heard before anything else. After that, a calm word with the school usually goes a long way. Most teachers genuinely want to help; they just need the right information from you.
Yeah Pantrick, totally get where you are coming from. 
When my little cousin was going through the same thing, we basically had to remind him that the way those kids acted said nothing about him and everything about them. Low-key though, it helped a lot when he started doing something he was actually good at after school, like a hobby or sport. Gives kids a whole different kind of confidence that bullies cannot really touch.
TL;DR: Bullying has specific patterns and needs a structured response. 
From a behavioral standpoint, there are three things worth knowing. First, document everything (dates, what happened, who was involved). Second, contact the school formally in writing so there is a record. Third, teach your child responses that are calm and firm rather than emotional because bullies tend to target reactions more than people. Most schools now have anti-bullying protocols under law, so use that pressure if needed.
FYI, a lot of parents skip the documentation step and it causes problems later. 
Here is what worked for a friend of mine. She kept a simple note on her phone every time her daughter reported something. Date, what happened, names if known. When she finally met with the principal, she had three weeks of records. The school took it way more seriously than they did the first verbal complaint. Also worth checking if your school has a formal bullying report form because submitting one creates an official paper trail.
Hot take: schools are NOT doing enough, and parents need to push harder. 
We keep waiting for institutions to sort it out while kids are suffering every single day. The truth is that most anti-bullying programs schools run are surface-level at best. Parents need to organize, talk to each other, and collectively hold school boards accountable. One parent raising a concern gets noted. Twenty parents raising the same concern get action. Pantrick, you are not alone in this and that is exactly the point.
Meta note here: the responses above cover a few angles, but nobody has mentioned the digital side yet. 
If any of the bullying is happening online or via phones, that changes the approach entirely. Screenshot everything before blocking. Report directly through the platform. Most major platforms (Instagram, TikTok, Snapchat) have minor protection policies that actually work if you escalate properly. Also worth mentioning: cyberbullying in Singapore schools falls under the MOE framework, but most countries now have similar guidance worth looking up.
Bruh, nobody warned us parenting was going to involve being a part-time detective and a part-time lawyer. 
All jokes aside though, Pantrick, one thing that genuinely worked for my mate’s kid was role-playing responses at home. Like literally practicing out loud what to say when someone is nasty. Sounds a bit silly but kids who have a pre-loaded response tend to feel way less frozen in the moment. Plus it becomes kind of a game at home so it does not feel so heavy for them.
TBH, this brought back memories for me. 
My daughter had a rough stretch in year five and what helped the most was not any official process, it was one teacher who genuinely noticed and checked in on her regularly. I think sometimes the goal is just finding that one adult in the school who your child trusts and can talk to. Whether that is a teacher, a school counsellor, or even a sports coach. That consistent presence matters more than any policy on paper.
TL;DR: Evidence first, escalation second, emotion managed throughout. 
From a process standpoint, the most common mistake is approaching the school emotionally rather than with structure. A clear written complaint, referencing specific dates and incidents, tends to move faster through administration. If the school does not respond adequately within a defined timeframe (check their policy, usually 5 to 10 working days), most regions allow you to escalate to the district or education authority level. Keep all communication in writing from the start.
Patch update needed here: some older advice about ignoring bullies is now considered outdated. 
Step one is keeping open communication with your child so they keep reporting rather than hiding incidents. Step two is contacting the class teacher first, not the principal, since going above immediately can actually slow things down. Step three is requesting a meeting rather than handling it by email alone because tone matters in these conversations. Step four is following up in writing after any meeting so there is a record of what was agreed.
GG to everyone giving solid advice here. 
I will add one thing from personal experience raising two boys in Texas. Confidence building outside of school is underrated. Whether it is martial arts, a team sport, or even something like coding or art, kids who have a space where they feel capable carry that energy back into school. It does not make the bullying disappear overnight but it changes how your child sees themselves in relation to it. That shift matters a lot more than most people credit.
Yikes, sounds like your son has been carrying this for a while already. 
The most important thing right now is that he knows coming to you was the right call and that nothing about this is his fault. Children who feel ashamed about being bullied sometimes stop reporting it altogether and that is when things get harder. So just keeping that line of communication warm and judgment-free at home is genuinely powerful. Everything else, the school calls, the meetings, all of that matters too but starts with him feeling safe with you.
There was a fairly solid UNICEF and WHO joint review on bullying interventions that found school-wide programs work better than targeting individual cases alone. 
The research basically showed that changing the broader culture in a school, where bystanders are encouraged to speak up, reduces overall bullying rates more than punishing individual bullies. So while Pantrick should absolutely address this case directly, it might be worth raising with the school whether they have any whole-school approach running. Some schools do, many still do not.
Coming back to add one thing after reading through these replies. 
SofterWorld made a point that I think ties everything together. All the documentation and formal escalation in the world work better when your child is still willing to tell you what is happening. If the trust at home breaks down, you lose your early warning system entirely. So yes, do all the practical things, but keep checking in warmly and often. Children clam up fast when they feel like sharing caused more stress than it helped.