Looking for a book... in Summer, summer, summer time!

When I was at school, in 1982, my teacher read a book to us. It was all about 2 children and the scrapes they got into. Throughout the book we didn’t even question the gender of the two main characters, just asummed they were both boys. At the very end of the book, one of them climbed onto the school roof for a dare - to ring the bell possibly - at which point the headmistress called “Get down (insert character’s name), you naughty girl!”

I remember that it caused quite a lot of active discussion and would love to know if anyone knows what the book is because I’d like to read it with my kids. Thank you!!