What are the prerequisites to join a flight School ? Is it different for each country?
What makes you think you can manage the exams for flight school if you can't even pass a high school exam?

They're at at least the same level of complexity...

But no, in general you don't need a minimum education level to start flight training. That said, if you want to get hired as a commercial pilot at some point, many employers WILL have education requirements without which they won't give you a job.

This is for EASA requirements, which are in use in the UK where you seem to be located. And of course individual flight schools may well decide to not take on people they expect to not have the capability to pass the exams, as having a high rate of failure on exams may reflect poorly on them. But that's not a legal requirement and would be a policy of the specific company in question.