Is there a particular rule for conjugating verbs in cases of semi-ambiguous subject?
Solution 1:
There's no "rule". If you want to emphasis the fact of the fish behaving as a unit, use the singular. You can always justify this by pointing out that "a school" is singular.
If you want to call attention to the fact that there are many fish in the school, all doing the same thing, use the plural. You can justify this by pointing out that Google Books has 1620 instances of "a flock of sheep graze", compared to only 102 for "a flock of sheep grazes".
As with the school and the flock, English speakers get safety in numbers by using the plural!