Just wanted your opinion

Hi all haven't been posting often (the bane called school) but today this happened and I'm questioning my decision so hope you can offer your take :)

So today we had to build a trebuchet or a catapult using materials given (i.e. ice cream sticks, glue, tape, etc.) in groups of 4. Each group gets a computer to look up instructions.

My group had prepared diagrams yesterday so we didn't need the computer (but one guy used it for coding. He likes coding.) at all. Then L (not his real name) came to our computer and entered a website with adult content (even though he's underage) into our search engine while we weren't paying attention. The rest of the group was testing our catapult outside the classroom and only I was there.

I shouted for L to close the website and he did. But I felt that what he did was extremely wrong (and not on his group's computer no less) so I reported it to the teacher privately. L got in serious trouble and for the rest of the day kept his head buried in his hands on the table. (I was feeling kinda bad by then) Not only that, the teacher called out me and a few other people (L's friends and his groupmates) to explain what happened.

At the end of the lesson L's good friend P approached me. He asked if I was the one who had reported L because I was near L at that time. I denied (I'm a fairly good actor) and made a huge show of being wronged. P tried to calm me down and explained to me that L 'meant it as a joke' and 'now's he crying because he's in trouble now'.

I felt kinda guilty. It didn't occur to me that L meant it as a prank and now he's in hot water because of me. But I also kept in mind that P could have lied.

When the day ended, L seemed back his normal bubbly self. (Maybe a little too forced bubbly-ness?) But I sought out my Philosophy teacher and told him everything. I then asked him if I had done the right thing. He said that even if L meant it as a joke, I had the right to protect my groupmates and report as L used my group's computer (and without the incognito function). If the school found that website in my group's search history, my group (and not L) would get in trouble. Thus, he concluded, I was correct to report this incident.

What do you think?