Why was I banned?

You should get an email about the ban. If you don't see it, check the spam filter.

It says for multiple accounts, but i think it was because we were on schools wifi

You need to reply to this thread if you want to get unbanned:

https://forum.callofwar.com/index.php?thread/16355-help-my-account-has-been-banned/

You should reply to the email. That will send your response to the accusation directly to the person working your case.

If you did not get an email you may post on the thread linked above.

The automated anti cheat system prohibits people from using several accounts in the same game by IP address, so it frequently detects players on school Wifi.

If you wish to play in a game at school, create one with the anti-cheat setting off or disabled. This will prevent the system from banning you, but you will not receive experience points or gain rank there.

There should be a way to flag certain wifi spots as school wifi, this way the problem is eliminated. But if there are multiple accounts on an IP they would be banned.