Right, I'm up again
1. My school day starts at 09:00, registration until 09:10, and then 5 lessons until 15:30. Each lesson is 1 hour long. However, some of those lessons are what we call 'study periods' - you don't have a lesson with a teacher timetabled there. During these periods you're expected to be studying and doing homework for your lessons, though at some schools (including mine) you are also allowed to leave the building during them so long as you're back for your next lesson or registration. If your characters are hyper-academic (like people say I am lol), they may choose to basically never leave the building during study periods and use them all. I also stay after school every day to study more, and my school is open until around 17:30 to allow that. If your characters aren't too academic (though you mention law, so they probably are), they may use their study periods to go and hang out at a shopping centre or something. Most people are somewhere in the middle and use some productively while using others to go out.
2. I'm going to assume you mean they want to study law at
university. College in the UK is another type of 16-19 education and an alternative to sixth form. Colleges offer vocational courses like hairdressing and social care, though they do also often offer A-levels too. Law is actually quite flexible on which A-levels you take compared to some other courses, though many aspiring law students choose essay-based subjects like English Language/Literature, Politics, History or the A-level in Law (only some sixth forms actually offer this as it isn't required to study law). However, 3 essay-based A-levels is a very high workload so people also often take, for example, 2 of the above essay-based subjects and Mathematics or a science to go with them. Also, in order to study law at university the relevant character(s) will have to sit an entrance exam called the LNAT. They will not be accepted to study law without sitting this, and you can find more information about it on their website:
https://lnat.ac.uk/3. This is entirely school-dependent and mostly depends on how many maths classes there are. The more maths classes, the less chance of them being together. Some schools with multiple classes will place students in a certain class depending on their other subjects, as subjects are arranged in timetabling 'blocks' (e.g. at my school, biology and computer science are in the same block, so their lessons are always at the same time, and engineering and chemistry are in the same block, so those lessons are always at the same time) and schools use these blocks to organise people into classes at larger sixth forms and it gets kind of complicated, but basically it depends on how large your fictional sixth form is. Smaller sixth forms are more likely to have only 1 or 2 maths classes, giving your characters a higher chance of being placed together. TLDR, if it's an important plot point that your characters are in the same maths class, it's entirely possible and nobody would think it's weird or unrealistic. Similarly, if it's a plot point that they're
not in the same maths class, also possible.
Please excuse any weird grammar as it is 6:42am

If you need any more info I'm happy to help, though it may also help if you told us more about the fictional sixth form you're envisioning (size, community, etc) so we can provide more specific advice.