How do I convince my teacher to let me out of school when I dont have lessons?

So I've done my A-Levels and I'm taking a gap year to do further maths A-level in 1 year. My school 6th form has let me back on for Y12 CP1 and D1 and Y13 CP2 and FM1 lessons except I've already done CP1 by myself. That means that if I have a CP1 lesson, I'm just going there to do my own work at the back. Also, because I only have 1-3 lessons a day, there are some days where I'm just sat in student study for 2-4 hours because we're not allowed to leave when we have frees. E.g. last Friday I had 2 CP1 lessons and 3 frees, so I basically went into school just to do my own work for the whole day. My CP1, CP2 and FM1 teachers were my old teachers from maths A-level and they don't seem to have a problem with me skipping frees but they told me to ask the head of maths for permission. He doesn't really know me because he hasn't taught me before but he's saying he wants me to stay for both frees and CP1 lessons. Which means that over 2 weeks, I have to go in for 44 hours for 14 (useful) lessons?? What should I tell him to let me off?

If you were doing the work at school, you would be doing the work at home. It wouldn't really make a difference - except at school, you will be doing the work

So I've done my A-Levels and I'm taking a gap year to do further maths A-level in 1 year. My school 6th form has let me back on for Y12 CP1 and D1 and Y13 CP2 and FM1 lessons except I've already done CP1 by myself. That means that if I have a CP1 lesson, I'm just going there to do my own work at the back. Also, because I only have 1-3 lessons a day, there are some days where I'm just sat in student study for 2-4 hours because we're not allowed to leave when we have frees. E.g. last Friday I had 2 CP1 lessons and 3 frees, so I basically went into school just to do my own work for the whole day. My CP1, CP2 and FM1 teachers were my old teachers from maths A-level and they don't seem to have a problem with me skipping frees but they told me to ask the head of maths for permission. He doesn't really know me because he hasn't taught me before but he's saying he wants me to stay for both frees and CP1 lessons. Which means that over 2 weeks, I have to go in for 44 hours for 14 (useful) lessons?? What should I tell him to let me off?

Keep quiet, and use the back gate or hole in the fence.
We had the same rule, but it was widely flouted and nearly everyone knew.
Never once was pulled up over it, though the unwritten rule was you were to keep yourself to yourself and not
do anything stupid, for instance no return to school if you had `sought refreshment` in the locality.
Happy days!

well if you can't tell him off, i suppose you could do supercurriculars to improve your application