Strixhaven leveling (WARNING SPOILERS)

Hey, so first off, I have no idea if this is actually the correct place to post this or not so please direct me where to if not, but does anyone know how the leveling system in Strixhaven: A Curriculum of Chaos in D&D 5E works? I am planning on DMing for the first time on a campaign and I don't know for certain if my players already have characters past 1-10, but if so, it says that throughout the campaign the characters will get to level up after certain events and I don't know how that works in accordance with the milestone system. For instance, say I have a level 5 elf with 10,568 exp, but I reach "Advance to 2nd Level!" on page 74. Would said elf gain a level's worth of exp, gain just enough exp to reach the next level, or have nothing occur at all? Or does something different happen?

Leveling at certain events is the milestone system. Don't worry about experience at all, just level up when the adventure (or you, the DM) dictates the level up occurs. 

If you want to run the adventure with existing, higher level, characters you could take the time to advance the module appropriately to match the character levels. It's not that hard to do but does take some time and can be a bit involved if you've not done this before. If you're happy to kick the adventure off with higher level characters and don't mind them strolling through the earlier parts of the adventure, you could: 1. Award the XP the milestone stands for. So, in your example you have a milestone saying "Advance to 2nd level!". Well to go from 1st level to 2nd level requires 300xp (Player's Handbook, p15), so that is what you could award each character. 2. Completely ignore the advancement until the character is just below that level i.e. in this case you would only advance 1st level characters and everyone else would get no experience awards. An existing 5th level character would only level up once you hit the milestone that says "Advance to 6th level!". Personally, I'd go for the second option as that way you will eventually have the adventure catch up to the level of the PCs.