Hey,
I just graduated from Glasgow Uni law with a first class honours. I got eight 1s in my S Grades and AAAAB Highers in 5th year, and in 6th year I got BB in Adv Highers and an A in a Higher. So you don't need all As at school to get a first class honours. My advice would be to concentrate on subjects which are essay based as these will give you the best practice for law at uni. It's not a disaster if you don't get the grades in fifth year, because you can do access courses at uni to help you gain admission. You can also study for a different degree and then go on to do the two year postgraduate accelerated LLB afterwards. Glasgow is a really great place to study so I hope you do well in your exams and get to go there!
Good luck. Let me know if you have any questions.