If your daughters are happy in school please please dont home school them. My friend hated being home schooled and I know I would never have liked it. Learning is not the only thing that goes on at school, you learn how to develop relationships with peers and progress socially.
Im not sure of the rules in Scotland but I think in England that they would still be required to take some of the standard GCSE's maths,eng etc. Also at 12 and 15 they are still very young to be limiting their choices in subjects as minds can change very easily!
Im not trying to judge so sorry if i sounded harsh this is just my opinion i hope it was some help.