PBA:
First, I wouldn’t call Kollel a “problem”. Mr. 80 yelled at us already in this thread.
If the ratio of non paying to paying is too high, then the school will run out of money. In general, Kollel type are defined as non paying from the get-go, even if they can pay more (due to professional courtesy & the sort, let alone lack of funds). In a small school/single school area (which is the norm for OOT), a large proportion of the students will be from such families (which tend to have larger family sizes), especially if there is a SEED or Lakewood Kollel, or a G’vir who is paying for the Kollel for his town.
In other towns (in town or larger cities), the schools are larger and the ratio is lower. Also, the “kollel type” tend to concentrate in the “frumest” schools (since there are multiple schools), so the other schools don’t have to deal with their non payment.