Asking a High School Teacher for a Letter of Recommendation I am an undergraduate senior applying to graduate programs in Computer Science (with math-y specialties, such as Machine Learning and Data Science), and am trying to figure out who the best people to ask for recommendations are. I've already asked a professor whose class I did well in and tutored for, and a former manager of mine who has a PhD in the same field. As for my third one, I'm thinking about two options:

a) My high school math team coach: Math Team was my life back in high school, and I feel that my coach is one of the few people who truly knows how much I love math and learning in general. In addition, he also has a PhD in the field (in fact, he and my former manager did their PhDs together!). The only downside to this that I can see is that he is not familiar with my work in college, but I think the fact that he knows academia and my own academic potential makes up for that. Unfortunately, I have read that recommendations from high school teachers are generally looked down upon, so I don't know how great that would be.

b) My current Machine Learning professor: Machine Learning is what I really want to do, and a recommendation for someone who actively teaches it and is involved in the field would be great. However, I am worried that since the semester is not over yet, he may not have an accurate feel for my academic ability (although I have been doing well in the class so far). Additionally, I do not know him very well - I have been talking to him a little bit about grad school and advice for a career in ML in general, but I don't know if that is enough for him to write a strong recommendation with.

Given this background, which recommendation do you think would be stronger?
Don't ask the high school teacher unless that's the only way you can get an enthusiastic letter.  First, asking someone from high school looks like an admission of weakness or decline, like you haven't impressed anyone in college as much as you did in high school, and that's not a good message to send.  Second, a high school teacher will have less perspective on how you compare with undergraduate and graduate students, which is the primary issue.  Finally, it's really hard to judge graduate school potential on the basis of high school performance.  For example, the math team experience you mention can be misleading.  Contest problems differ systematically from research in that they are guaranteed to have short, elegant solutions and they have strict limitations on what sort of background might be required.  These constraints are often comforting to students, and I've known people who loved math contests and were great at them but didn't enjoy or weren't particularly successful at research.

I'd recommend talking with your machine learning professor to see whether he thinks he's in a position to write a compelling letter.  If so, then go with that.  If he is reluctant, then you should choose someone else.  That could include the high school teacher if necessary (better a recommendation from a high school teacher than from someone unsupportive), or other options you haven't mentioned.