I'd like to use a single word or an idiomatic expression to express that the school's breaks are well-spaced and not too many. Is there an adjective that mean just the appropriate number?
You could use optimum/optimal
best; most likely to bring success or advantage; best; the best or most effective possible in a particular situation (Cambridge)
As in this medical article:
Is there an optimum number of night shifts?
You could say that the school's breaks have optimal frequency or distribution.