DP because large post.
One year and a half late in answering but the easiest way is by defeating Tentacle Grapes on floor 3-5. They are weak to Fire and even the Protagonist has access to a Fire spell (Ignite, 200 Destructive prof, requires a Secret Note from Jane if it is your first playthrough as the Protagonist initialy only is proficient in Debilitating and Physical magic - or you can fry them with Val or Sarah's basic Fire spells if you don't want to grind).
(You can still quickly grind off tutorial Demoman without any side effects but at this point MC and Val only have access to two schools of magic. After this, if you want to stay at level 1 but max out your profs, from memory, it takes Trust+Obedience=35 to manually control a party member so if the third one is annoying, use their dead body as a dummy. There isn't any downside to having the MC level up a bit, bear in mind stat gains are extremely random, it's almost as frustrating as gaining a level in D&D and trying to minmax while the Game Master is facepalming and telling you for the nth time that
no, you do not minmax at tabletop D&D, if your dice rolls keep wrecking your character, fate is inevitable. Of course, you can both save and "fix" those stat gains in Adventure High but they'll reset to default and level 1 on a NG+, so don't waste too much time rerolling level ups.)
If you're a completionist, it's very easy to make sure you have everyone in your party and they also are proficient in all four schools. The more advanced the notes are, the more proficiency they give if you want to save time on spamming spells. It's recommended to give Val and Sarah the first Support notes as First Strike is invaluable and Val's heal isn't all that bad if you can't Flirt efficiently (or hate her).
Valerie: Needs Physical and Support. (Take one of each lesson with her and Sarah. If Sarah knows Passion Heal and you have Kate, take Kate for the Support class instead.)
Sarah: Needs Physical, Support and Debilitating. (Support can be omitted if you date her and take the lead, she gains 10 for Passion Heal. Take one of each lesson with her and Val.)
Molly: Needs Physical, Debilitating and Destructive. (Take one of each lesson with her and Julia.)
Julia: Needs Physical, Debilitating and Destructive. (Take one of each lesson with her and Molly.)
Ashley: Needs Support, Debilitating and Destructive. (Technically. Take one of each lesson with her and Cassandra.)
Cassandra: Needs Support and Destructive. (Take one of each lesson with her and Ashley.)
Victoria: Needs Physical and Support. (If Kate still needs to bootstrap Support, have Kate accompany her, otherwise use Derrick.)
Kate: Needs Support. (If you didn't use Val, go with Victoria or Derrick.)
Derrick: Needs Support and Debilitating. (See above.)
Because Destructive classes literally are the only ones that have priority over any ongoing task that requires Jane Smith, ensure Cassandra, Ashley, Julia and Molly all have a non-zero prof by the last note (20F). You can get your entire party fully proficient by floor 17 - Cassandra and Molly/Ashley should take priority for the floor 8 note (Julia won't join until floor 16.)