Translation:There are not very many fish in the school.
'Plurimi' can mean very many as well as most.
You could also say 'permulti' for very many.
When "sum" indicates the existence (i.e. There is, There are, There was, etc.) it should come as the first (or second) word in the sentence. Otherwise it is the copula.
"Non sunt plurimi pisces in ludo": "There are not very many fish in the school" "Plurimi pisces in ludo non sunt": "Very many fish are not in the school."