If people need reasons to uphold a masquerade for the duration, here's a few.
- Anybody going to school will have rumors spread about them pretty quickly if they go off-script. This probably isn't "alien invasion" levels of bad, but it can be embarrassing and troublesome for a long time.
- Leo needs to attend school regularly, and do well, to uphold his bargain with AEGIS.
- Jason Quill must make public appearances, and his "brand" (the Foundation, any charitable work his family fortune is providing for, etc.) is vulnerable as well. I see no reason not to tell his human assistants and staff. But...
- Jason also has access (via biometrics, but also I assume by virtue of just being Jason) to his household's computer systems. The smarter AIs might refuse to accept commands from somebody who claims not to be Jason. Not-Dad in particular has already shown that he won't grant access to an important and useful thing simply for this reason. Retaining access to these AIs is probably important.
- It's possible that Harry's family has committed him to after-school appearances or other activities for charitable purposes, e.g. mall openings and the like.