I would assume pranks are venial sins?
So my father was washing his car and listening to a cassette player when I got home from confession. He told me not to lock the garage door because he wanted to be able to access the cassette player or something. I thought it would be funny to lock the door for a few seconds then unlock it. I had no evil intention. I only intended to get a few laughs.
The only thing that made me start wondering about the morality of pranks was my father’s retort that I needed to go back to confession (keep in mind that my father, while Catholic, is not religious whatsoever.) However, he was most likely joking.