Hello,
So essentially, Glasgow Cali doesn’t actually specify whether the Highers should be achieved in S5 or S6, meaning that as long as you meet their minimum entry requirement, you should be able to guarantee yourself at least a possible place in the course. Only thing is, they may not accept resits, so you might have to call up the university and ask them about this. I think AH biology is quite good, teachers for some reason don’t recommend this but many uni students do as it gives you helpful insight into what biology at uni level will be like. All in all, it’s really up to you, if you have a career advisor at school maybe ask them for their opinion on what you should do.