I witnessed a teacher get tickled by another student once...a Nun, no less.
Seventh Grade, Sister Beth. Sister Beth, was not horrible looking, as Nuns go. She was not fat, but kind of athletic, maybe 5'8 with HUGE calves and big feet. Back in my day the Nuns all wore habits that included a skirt that came below the knee, nylons (usually tan), and a conservative shoe. She kind of looked like a farmgirl-next-door with brown hair but never any make-up.
On this day, we were working on some project that had us all sitting on the classroom floor in a large circle. The desks were pushed out of the way, and Sister Beth was directly across from me. It seemed that all of the girls were on one side, boys on the other.
The first thing that got my attention: Sister Beth slipped out of her clunky black shoes before she sat down with us. He tan-stockinged feet were on display. She tried to tuck her feet under her, but her long skirt was in the way. Sister Beth was sitting next to Kathy, a girl I'd known since first grade and an annoying "Teacher's Pet". She sucked up to every teacher and got away with it. Her and Sister Beth were working on their part of the project together, and Sister Beth was really trying to act like "one of the girls", bonding with Kathy.
I have no idea why, but I remember Sister Beth was talking to a girl on her other side (her feet were next to Kathy) , when Kathy ran one finger down the Nun's pantyhosed sole. Sister Beth about jumped off the floor, much to Kathy's delight. No one else seemed to notice.
Soon, the bell rang....it was the end of class and the end of the day. Kathy and Sister Beth were still working, so I wasn't going anywhere so long as i had a view of those big stockinged feet. Kathy seemed to finish and started to get up; Sister Beth was still focused on her work...Kathy struck again.
She shot one tickling finger at Sister Beth's feet; the Nun jumped again, and let out a laughing "Whooo" in surprise. Kathy then grabbed one ankle and spider-tickle attacked. Sister Beth went into instant redfaced hysterics. She tried to get up but was a bit trapped by the angle and her long skirt. Kathy had one ankle/foot trapped beneath the other, and was scraping both ultra-ticklish nylon soles. The nun finally slapped (they were good at that) Kathy hard on the arm and that ended the tickling.
I still remember Sister Beth struggling to get up in that skirt, and then standing their in her stockinged feet, furious. I did spend the rest of the school year trying to figure out a way to get my hands on those feet, but never came close to getting a chance.