You can have them brush their teeth to get Confident and use its boosts. If you're out to gain Skill as quickly as possible, have a nicely decorated room and make sure your Sim has high needs. Angry gives a +20-40% boost (if enraged, which is a dangerous state) but Happy is a much safer choice, given it gives +20% Skill gains for ALL Skills and +40% at Very Happy. The Ideal Mood for leveling Mischief is technically Angry, but Sims will get better Skill gains while Very Happy.

Slap 'em Silly can be accepted or rejected like all other Interactions You can now set your skill level with t_skill_level major_mischief x (fill in 1-10) - we also have a full listing of Skill Cheats and a master list of Cheats in The Sims 4 ->

Press Control + Shift + C then type testingcheats on and press enter.