It’s not about not letting them make cards in school, it’s about not going out of your way to show any importance to these holidays (i.e dressing them up in colours and sending them to school so that they look like everyone else). If they are making cards in school you can’t tell them no and make them just sit there and watch everyone, that’s cruel. As Niksik said it’s a very fine line. I believe that as a parent you should teach the that we dont follow what these ppl do b/c we are muslim.
And you are absolutely right. We have no control over what they do in highschool, that’s why it’s so important to instill these values in your children at a young age. If done properly then they will understand that we as muslims don’t take part in these activities.