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.