This girl can get Christian teaching elsewhere. She can even pick up a bible and read it all on her own. The fact is, the 'parents' agreed to these rules, then violated them. The school isn't teaching a good lesson to anyone if they just allow rules to be flaunted.
Everything you say is certainly true, but there is no scriptural authority for it. This was my original point. The school is certainly welcome to do as it pleases, but to claim scriptural or even scripturally supported moral authority for their actions is dishonest. There is no support in Christ's message for the passive evangelism you suggest. I expect Jesus would have handled it differently.