As far as being HMS friendly, as many other people have said, I'm not sure their is a perfect job, however with regards to teaching/teacher's assistant, it's not the worst career out there!
I'm a student teacher at the moment and after my third placement where my university 'forgot' to mention my EDS III

, I've learnt to adapt to things, including teaching PE!
I know most people expect teachers to spend all day on their feet but I haven't had to do that! I sit whilst explaining activities to children on the carpet, and usually I set up a 'base' where children can come and ask me for help, if I'm having a bad legs day. If I need to move around, I walk around the classroom to check the childrens work rather than sitting with the children coming to me. Having my 'little helpers'

is also a benefit, especially in PE as the children love being the ones to carry and set out equipment and even lead warm ups and some activties, which helps me a lot if I'm having a bad day. Little things like support bandages (which kids seem to love hearing about!) and having different size pens to interchange when writing on a board or marking work, help a lot! The advantage of being a teacher is that's it's your classroom and as long as you are providing the children with a good education and a safe, fun environment, you can set it up and teach how you want, making the classroom flexible to your needs!
The only thing I've found tiring is the paperwork and preparation for lessons as that requires a lot of sitting down

but cups of tea and streching help with that

Although teaching can be tiring, it's also really fun and offers more options to move around or sit down when necessary, than other jobs do!
Sorry if that's a bit of a mouthful - I tend to write as though I'm actually talking!
Hope that's helpful to anyone considering teaching!
Nicole