Well it all depends on whether your manager is exceptioning the break or not on payroll.
If they are, then they'd of told the system you're only taking a 15min break, so it'll pay you for the other 15.
If not, then it's auto deducting 30mins due to your age, meaning you're missing out on money.
Technically your manager is right though, a 4hr shift does equal a 15min break, but that's based on a colleague over 18yrs old. As you're younger, different break rules apply.
It's news to me that a 17yr old is allowed to work in an Express, in my old one they only employed 18+ due to having to go on tills serving cigs/booze.