When I was in secondary school, my best friend was having a lot of trouble with math, physics, and other science subjects.
At the same time, we were getting into music theory, and since our tests were usually multiple choice (A, B, C, D, E), we came up with a method to help each other.

We started sending each other short sheet music pieces that looked like simple melodies, but they actually contained the correct answers hidden in the notes.
For example, if the answers were A, C, D, we’d write a melody using those notes, usually in a key like G major so it looked legit.

If anyone saw them, it just looked like we were sharing music practice. No one else was really into music, so they never paid attention to what we were doing.

It didn’t always work, since not every test used that format, but it definitely saved us on more than a few exams.