not a teacher
There is a slight ambiguity.
It depends on whether you mean:
1) an old school style of bag closure. (old school = traditional, old-fashioned)
2) an old schoolbag style of closure. (in the style of a schoolbag from past years)
You could word the appropriate option as I've suggested above.
"Schoolbag" is sometimes written as one word and in this case it would help with the clarity.
Interested in Language