I can think of one time they were implied to be in the same class, though some fanwanking can save the day.
Here’s the set up: Sally and Linus went on at least one school trip together–a trip that didn’t include the Charlie Brown/Lucy set. The one I remember involved a trip to a farm, where Linus got caught in a bizarre triangle between Sally and “Truffles,” the girl Linus met a few years back. Linus was so upset by the girls’ bickering over him that he climbed up on the roof of the barn, and eventually was saved by an “air rescue team” consisting of Snoopy (doing his whirlybird impression) and Woodstock (the pilot).
However – now here’s the kicker: Later, Sally gives a report in class about the incident, highly melodramatic, of course, and at one point screams out: “Who would save my sweet babboo?!!” A speech bubble pointing offpanel gives the response (presumably from Linus): “I’m not your sweet babboo!!”
Anyway I can fanwank the Linus/Sally field trip by saying that Linus was taking several upper-grade classes with the older kids because of his intelligence, but he’s still in the lower grade (say, 3rd grade or so) and maybe the class trip was for the first through third grades. Which explains why Sally is also on the trip. And perhaps Linus was only in the classroom where Sally was giving the report because all the kids who took partin the field trip were present, regardless of grade. (Yeah, it’s a stretch, but…)
Of course even my above fanwank doesn’t make sense according to strict continuity, because Sally should actually be five years younger than Linus (who right after Sally is born takes pains to calculate how old he’d have to be in order to start dating her; I think he settles on something like 22 and 17). I can’t think of kids five grades apart taking the same trip. Plus, this means Linus would have to be at least in the sixth grade (we saw Sally leave Kindergarten so she’s at least in the first grade), which puts Charlie Brown et al. in seventh grade, which is utterly preposterous.
As long as we’re finding inconsistencies: I also seem to recall Linus and Sally being together on a field trip that seemed to have the purpose of teaching the kids about different vocations. The weird thing about this is that one of the job sites was a barber shop, where Sally acted as if she were completely ignorant of the whole, um, barber shop millieu. And yet of course it had been long established that Brown pere is a barber. Not that continuity was hugely important to Schulz, but it’s hard to believe he’d forgotten this detail, considering how often Charlie Brown visited his dad at the shop, and there were a series of strips where Schroeder was talking to Charlie Brown about how Schroeder’s dad was kvetching about the “high price of haircuts” and Charlie Brown pitching a sarcastic fit over how luxurious his dad’s life was.
Yeah, I have a little bit too much knowledge about Peanuts.