I was under the impression that once you PASS the first exam you have 18 months to pass the GST & get PPL.
If an exam pass is invalid (18months old) you re-take that one & the 18months then starts from the time you passed the second etc until you pass the GST.
I don't think it is a legal requirement to pass air law or any exam pre-solo but many Flight schools make it their rule. So if that's "Their" rule then yes.
Short answer: check with the school.