Гл. м. could be "главна мета" - main target
The question is who was the main target.
Media say it was possibly the history teacher, with the perpetrator having a poor grade from history. Others mention that it was a case of bullying that drew the student over the edge.
The student happens to be the son of a highly respected radiologist, and the school he was going to is probably one of the most posh elementary schools there - apparently most class is conducted bilingually in French and the school itself has the support of the French embassy.