They are going all out to hide their relationship, that is sad, just because they fear being teased at school for a couple days when even if someone tried they just would have to be even closer and everyone would stop, because if it happens is to get a specific reaction.
If they just casually brought it out like "oh, can't do that day, I'm on a date, what? yes we are going out from the last school year, never told you? we wanted to keep it a secret for a while and forgot" it would get ignored pretty quick, or if something big happened in school or another few couples popped out it would just feel like is that time of the year.