Students come and go at D&D Club all the time. I just keep them all at the same level; nobody cares and it's much easier to track, create encounters, etc. And it sounds like the purpose of this is to give them a taste of the game. I definitely concur with using the most current rules; for me the main imperative is to make it easy for them to spin off their own games once they get the taste, and you aren't doing them any favours by teaching them a niche system or a system from your youth. The current rules will give them, by far, the system where they are most likely to find other players (better still, use DDB).
I don't bother with experience points; most new TTRPGs don't use them and they're a PITA in this kind of situation in particular. Just have a campaign level, and if that means some players level up after missing a few games, or join with a 4th level character, then that's fine.
I don't really understand the claim that the 2024 rules can't support open table play; it's not hard.