My kids have always known Santa wasn’t real. We just nonchalantly talk about which adult is going to be santa this year. It’s like playing pretend, and doesn’t make the kids any less excited (but does remove the awkwardness of explaining why it’s ok that a strange old man you don’t know is allowed to come into the house while everyone is sleeping because he is giving you stuff, but other strange old men trying to give you stuff shouldn’t be trusted).
For the telling other kids at school thing, my sister would say that it’s not her responsibility to cover for other parents lying to theig kids. We would each be honest to our kids and let other parent handle their kids.