I agree with that the shirt should not be allowed. Similarly, neither would a shirt with something like this:
But it would have to be dealt with tactfully and appropriately. If it was the first time the student wore anything like it, I would expect the principal to let him know why it was inappropriate and why it was not allowed. If he came back the next day wearing it, then the suspension is warranted. At that point, you are intentionally wearing something that some people find offensive and you are intentionally breaking the rules.
You can share your beliefs without insulting others. Telling people that they are wasting their lives if they do not believe in Jesus is insulting them.