My in laws would have had DS living at theirs given half the chance. I’d never met such pushy, self centred people as them and was naturally quite submissive when I met/before I met them.
They made everything into a fight and I still hate them for it now. ‘We want to wean DS on custard,’
‘No thanks MIL, we don’t want his first food or even first six months of food to be custard.’
‘BUT WE WEANED OUR SON ON CUSTARD. HE HAD CUSTARD EVERY DAY FROM THREE MONTHS OLD AND HE’S FINE.’
‘No thank you MIL, it’s really kind of you to offer to help but no thank you.’
‘Now you’re here for Sunday dinner, we’ve made some custard for DS. You eat your meal and I’ll feed him this custard.’
‘No thank you MIL.’
And on and on until I felt forced to shout, because they were so relentless, and then they cried down the phone to DP.
They also pushed for overnight stays since around two weeks old and, honestly, I wasn’t ready. By the time I was, when DS was around 2.5, they’d pissed me off so much that I delayed till 3.5.
I wouldn’t be letting my DC go with them full stop, OP. Grandparents are nightmares if they don’t stay in their lane, and my in laws made my first year as a mum needlessly stressful.
Regarding overnight stays, I’d be stopping those if possible. You say your classes run over bedtime, so is it like bedtime is 7pm and you get back at 8pm? If I were you I’d be telling your parents that they are baby sitting at your house and then they get their evenings back when you return, unless you genuinely enjoy that night to yourself - but I’m inclined to believe that your parents pushed you into it to suit their own agenda.