To be honest, I'd be pretty upset. That's some serious stuff to lie about. And not only that, but with the report card thing, she's encouraging the kids to keep things from you and lie to you as well. I would never tell my kids it was okay to hide the report card from dad, you know? I'm not saying she's a bad person, but that was a bad decision. You know what a situation like that tells the kids. "If I do something bad, mom wont punish me for it and dad will never know because they don't communicate like they should". Which in turn leads to not trying as hard as they should at school. I have to admit that a lot of times when I was getting good grades it was only because I didn't want to get grounded or yelled at lol but I thank my mom for that now. Seriously, if I thought I could get away with it, I didn't do anything in school. I would get straight F's if I knew my mom thought I had A's and B's haha. I'm smart, but lazy. And I hated school, so my mom had to keep on my butt or I would have failed.
I think most kids are in a way like that. They need to be motivated to reach their highest potential. And the idea with the van payments... it sucks, but you gotta be honest because something like that affects everyone, you know?
So if you're asking how I would feel, I would be really upset. And THEN - I would directly afterwards stop and question myself as to why this happened. Like maybe she didn't tell me because she knew I would get mad. Maybe she was scared and worried about it and didn't want the confrontation. Maybe she knew I wouldn't be nice about it? I get like that with BF sometimes, there will be the stupidest things he wont tell me or will lie to me about (kind of) and I'll get really mad he kept it from me but after we talk about it, it was completely harmless and he just didn't want to say anything because I would over react for sure. I just have to think about it and wonder how I would feel if I was him and I try harder not to get mad over stuff I shouldn't or be more understanding about stuff that's hard to deal with.
Not saying that any of that applies to you, just trying to help. It seems to help me. None of that makes it right for her to decieve you like that, but it could help you deal with the fact that she did. Just dig deeper. Cause - why wouldn't my fiance tell me our kids' grades? What's the big deal? You know?
I don't know. But I hope you feel better... it's really not so bad once you think about it.

I bet it will pass.