Having a Junior High School reunion makes sense for someone of Thelma's generation as about half of the students at the time(late 1930s) stopped attending school by the 9th grade.
While today most schools have a 5th thru 8th grade "Middle" School, in Thelma's time there were 7th thru 9th grade "Junior" High Schools. Thelma and Clayton were most likely in 9th grade when they went to the Jamboree.
Being invited to a Junior High School reunion as opposed to a Senior High School reunion makes sense canonically since it was established in Season 4 Episode 1 that Thelma never graduated High School.