I haven't had to deal with this in awhile because my kids are old enough now that they don't have a lot of parties or playdates, but young enough that they don't have an active social life. So they don't get grounded from things, they just get their computer taken away these days. When they were younger, I might cancel an outing, but usually that was when we had planned a trip somewhere ourselves and they weren't behaving. The trip might get postponed a day or so, or if it was a reward, it might be taken away. I wouldn't impose a grounding when we had plans to do something right then, like go to a birthday party, or what have you. Now if my child was sick or just in such a bad mood that going would have been bad, I might cancel, but usually I'd wait out the bad mood, talk about the issues and then go to the event. I don't really think I felt punishment was needed in the early years, but if I was trying to impose a negative consequence, it would usually be the loss of another privilege or possibly future outing--like if they wanted to go for ice cream or something. The most common situation was that my daughter would be in a really bad mood and not behaving well, and we might cancel our weekly playgroup outing, but no one was particularly waiting on me, and it was better for my daughter's emotional state to do this.
One time I was pretty ticked off at my older daughter who was about 6 or 7, and was thinking of not letting her go to a birthday party, but then that seemed like a rude thing to do, and really, what I wanted was not to impose a punitive thing, I just wanted her to do the thing I asked her to do. And, really, it turns out that she's usually OK with not going to a social outing--she didn't really care if she didn't get to go to the party, she didn't see it as a reward for good behavior, it was just part of our life. Choosing not to go to that would have been an arbitrary punishment that didn't really teach a good lesson, and would have the added negative of not following through with a commitment. Sometimes I think the canceling of plans as a punishment is really just the easier way to do things for the adult, but it's not necessarily the best consequence or punishment. I was also trying to teach my children to be respectful of others, and we don't like when people cancel at the last minute. The only reason to cancel would be if they child's behavior was so bad that she couldn't go and be a good guest, but that was not an issue. And at their current age, my kids would far rather I keep them from a party than take away their computers for a week.
Now with older kids, I feel like it's a different situation. Like if your teens are just doing egregious things, you might have no choice but to really crack down on all their activities. But, given the little I know of this situation, I wouldn't have my child cancel weekend day plans with other people because she was late for school; this isn't really teaching her responsibility in my opinion. I'd probably make her go to bed earlier or do other things that helped her learn to be on time or what have you. But if she really wanted to do the group thing, I suppose this could be a negative enough consequence to stick with her. Somehow, though, I have a feeling she'll continue to have problems, especially if her cellphone or facebook privileges aren't ever taken away. I don't really see how this kind of punishment is going to help her become more reliable and responsible, but that's just my opinion, of course. :)