The way the class system and skill costs are set up, it's better to start out as one of the 'pure' classes (Fighter/Rogue/Scholar), build up the primary skills of that class, then start branching out into the skills of another class until it's cheaper to be one of the 'hybrid' classes (Adept/Scout/Templar), and then make the switch.
If you want to be an Adept, the general recommendation in our chapter is to start as a Rogue, then when you've got some Backstabs and other skills under your belt, start getting spell slots until it's cheaper to switch to Adept.
Spell slots have to be purchased individually in a pyramid-ish fashion and are per-day skills, but low-level slots are fairly cheap; Fighter and Rogue skills are gated behind Weapon Proficiencies and/or Backstabs, each of which requires a not-insignificant BP investment but grants static damage you can swing all day long.
Which sounds more appealing is going to depend on you and you preferences, but remember that BP is earned quickly early on and then slows down significantly by about level 15-20, so buying high-cost skills early and saving cheaper skills for later will keep you feeling like you're making progress towards your goals and give you new toys to use when your BP gain slows down.
Beginning as a Celestial Scholar, you'll tend to run out of spell slots very early on in the day unless you ration them meticulously, which isn't necessarily a bad thing, but can make your first few games a bit on the boring side if you want to do a lot of combat or don't have a supply of Scrolls you can use; beginning as a Rogue will give you a Backstab and a cheap-ish weapon skill (and maybe also Florentine, if you want to wield two weapons), so you can swing for 4 from behind and contribute more to combat over the course of the whole event, but you'll be a bit delayed in learning any spells.