I left school in 1970 without any qualifications and went on to get my CAA engineer's licence. My worst subject at school was maths but I can carry out any calculations pertinent to the job.
If you know your tables and basic arithmetic, that should be sufficient. You'll need some knowledge to mark out metal, determine rivet pitch and use vernier measurement tools. I don't know what the licence exam questions are like nowadays. I think the only one I was asked was 'what's an eighth of an eighth?'
Then again, things might have changed.
I'll be interested to see other viewpoints.