Am I being taken for granted, or is this just what being a grandparent is today?

This doesn't sound at all normal. You are that child's parents and you shouldn't be. If you want it to end, you need to just not be available sometimes, set times you aren't available to them, or have a conversation and work out a plan of what you are willing to do. You're completely reasonable in wanting to be the grandparents, not the parents.

I am assuming your daughter is a student, but she is behaving like one not as a mother and is neglecting her son. Does she live with you?
If you continue like this the demands will escalate.
Start by planning a weekend away; give her plenty of notice but stick to your plans no matter what objections she raises. Continue, and become unavailable in the evenings due to prior commitments.
Where is the father?

First off, what a lovely thoughtful question you ask. You clearly adore your DG. And yes I do think grandparenting has changed. However this situation has probably crept up on you. I think you should talk to her about how you are feeling but tread carefully as she is probably unaware of how you are feeling. And you don't want to create hostility whereby she makes totally different arrangements, as you're likely to feel worse about the situation. Start with a couple of tweaks/boundaries that you and your husband want to change. And let her know that you will build on them gradually. Be honest and say that you are both finding it too much and be clear on wanting to stick to some of the agreements you want to make. You actually don't need to say what you want to do with your time, that's your business and she will have to respect that. But you probably need to do it for her sake too as she does need to take more responsibility with her son. The lines can become blurred and she does need a reality check on committing to the upbringing of her son.

“We agreed to help with childcare for classes,”………. then you need to have the conversation with your daughter and remind her of the original agreement. You also need to ask her what her long term plans are because if you don’t set boundaries now you might find yourselves raising this child completely with no responsibility taken by his mother.

Does she live with you? How does she fund her social life?

Totally out of order, unfortunately for her if she chooses to have a child young they come first, your daughter is not a single student she’s a mother. Continuing helping with the classes and a few outings but be unavailable for her partying
Where’s the father in all this ?

I'd also bear in mind that because you have made yourselves so available, it might be making her feel a bit inadequate as a parent. She probably sees you doing all the right things with her son and may well think she'd never be as good as that. So in a way your role now is to encourage her to bond, engage with, and take responsibility for her son. It is important for her to do this.

You must set boundaries. You are still young but you and your DH have your own lives. I understand it is difficult for your DD especially if she us a single parent? Single or not she is a parent.
Your DD is still in ' child' mode and still thinking 'Mum will help me out' that only goes so far !! Tread with care but decide how much child care you can offer and stick to it. Be cautious with what you offer, you can always add to the arrangement but it may be difficult to withdraw your help.
Good luck , you are grandparents , you are not vo parenting!

you might find yourselves raising this child completely with no responsibility taken by his mother.

This happened to my nephew and his step daughter. He and his wife ended up practically raising her son for her. Fine while they’re young. Not so fine when they grow up and start to analyse their feelings towards all the adults in their lives.

Tackle this now while you still have some control over it.

Do you both work? Does your daughter and grandson live with you? Where is the child’s father?

You are obviously doing too much, so decide how much help you are willing to give and tell your daughter as soon as possible. You have your own lives to live and the sooner you remind your daughter of this, the better.