The correct answer is "It depends" really, since no one here knows anything about you, your temp sensitivity, etc.
Personally, I would always opt for a thin wicking layer plus a medium/heavier outer layer rather than just a single heavy layer. I find it's more comfortable/flexible to have two layers than can "slide" separate from each other. For the first dive I'd suggest the thin layer plus the lighter of the other two. Water isn't too cold this time of year (and you won't be terribly deep on a drysuit class) and even if you are a touch chilly it shouldn't be too bad. If you end up really/uncomfortably cold on dive one (or a set gets wet) you can swap for the heavier set for dive two.
Note that some instructors will get pissy if you change undergarments between dives as that will impact your weighting. Too bad for them... if you need to change undergarments or other config things that change your weight, so be it.
However, do be sure to discuss your thinking before dive one, and also tell your instructor if you do change anything between dives. It's a real pain to have a student corking/sinking during the second dive (after you've really dialed their weighting in on dive one) only to find out that they changed something between dives without telling me and/or making any adjustments to accommodate.
