Unless you are super strict, even payment one month in arrears can easily become a whopping amount unpaid. To give an example:
-parent is a student so getting funding from their college/Uni. Who pay in arrears...
-even though we're normally payment in advance we accept this, trying to be supportive. Parent has 2 children, full time.
-standard terms are 1 month in advance, 28 days notice.
Children attend for first month. By end of month, no money received. Contract is with parents, college won't talk to providers. Chase parent, claims they'll talk to college. Person at college is on holiday, person at college says their claim's been processed, yada yada yada. Every time you're in touch it's a couple of days of "I'm chasing" before you get "oh there's another form"...
Eventually, 7 weeks in, parent pulls both kids with no warning.
College eventually talks to us and advises the parent started their course but very quickly stopped attending. The college paying anything is contingent on attendance...we're getting zip. Parent is unemployed, no point in chasing through courts.
7 weeks not paid for, plus 4 weeks notice not paid - times two kids.
I'd love to say this sort of thing is rare but you can see from all the posts above that it's really not. We've had variants on the above with colleges, unis, tax credits, "delayed" childcare vouchers, the job centre...which is why, like most, we're now payment in advance regardless of the situation.