No personal experience but I don't think you're overreacting. Unfortunately, various state governments have caved on the religious instruction in public schools by allowing undertrained volunteers in to preach to kids as part of RE. However, it should ONLY be in RE, which I think is half an hour a week. There were a few posts about this topic recently inlcuding the fact one organisation has pretty much been given a monopoly on this.

Definately complain to the principal. Religious instruction belongs in churches/temples etc or at home, not in secular schools.