Evolution is indeed just a theory - the one every scientist sees as the most likely - but still a theory.

Edit (reply-limit): Thanks readerrr and cremno, valid points, will consider it in the future.

What I meant was, there have been theories before the current theory of evolution. For example Lamarck's one, which is long considered false but lately some aspects of it are reintroduced. If we would punish everyone for denying evolution we would be unable to advance it.

And of course this has nothing to do with teaching children biology. Of course children need to be taught evolution there.