Employment rights query (scotland)

Work part-time as a lifeguard and my shifts are shorter than everyone else’s. A colleague approached me the other day about swapping days ie if I do today they would do next week for me. I am currently doing his shift. But, his shift is 2.5 hrs longer and my supervisor expects me to work this without any pay whatsoever because it was «*an agreement between two individuals «*. I assumed I would get paid overtime or alternatively work my own shorter hours but apparently not. Anyone know what is meant to happen?

Also it is 8.5 hours and I heard anyone under 18 can only work 8 hours a day

You should be paid for all hours worked. So if you work a longer shift then you should be paid for those extra hours. It will probably be at your normal rate (for a higher overtime rate) but you should be paid for the whole shift. You swapping shifts with someone is not an excuse for them not to pay you.
I'd speak to a manager to sort it out. Don't let it go.

I may be wrong but I think you can work 8.5 hours a day. As long as you get your required break. And you aren't working after midnight or before 6am.

Thank you. It didn’t sound right to me but it is good to get confirmation.