In Bridge to Terabithia, why didn't Jesse invite Leslie on the perfect day with Miss Edmunds?

This is a great question, because the text offers a few possibilities that explain why Jess doesn't invite Leslie to go along with Miss Edmunds.  

I think one reason is simple, polite etiquette.  Miss Edmunds invited Jess.  If she wanted to invite Leslie too, she would have done so.  It is not Jess's place to invite somebody else along.  

Another reason is that Jess has a bit of a crush on Miss Edmunds.  First, the text tells readers that she is a fairly attractive teacher (at least in Jess's eyes).  Second, Miss Edmunds is the only teacher that encourages Jess to pursue his passion for drawing and art.  Spending time alone with Miss Edmunds is huge.  Jess doesn't have to share his time with his crush with anybody, and she will be able to focus entirely on Jess.  

I think that one-on-one time with an adult is a huge draw for Jess as well.  He comes from a family of five kids.  His parents simply do not have time and energy to devote one-on-one time to each child.  Leslie is the only child living with her parents, so spending quality time with an adult is something that happens to her all of the time.  Jess wants to experience some of that for a change.  

I think a final reason for not inviting Leslie is that Jess doesn't want to have to face Leslie and tell her that he doesn't want to go to Terabithia while the river is flowing so violently.  He's afraid of what Leslie might think about him, so a day alone with Miss Edmunds lets Jess avoid that conversation for an entire day.