What’s the best time for a high school student to email college professors for research positions?

Well, you will likely have a hard time being able to do this during covid. Professors most likely won't have anything for you to do virtually. Most of this type of research is done in labs and typically you'll only be able to observe or do basic tasks to help out. So I'd wait until the covid situation gets better and you can be in the lab. My local universities had in fact suspended all these high school research positions. You can check with your local universities/professors but this will likely be the situation for you. For these research positions though, people typically do them during the summer when they have more time. Its quite hard to balance school work while helping in a lab. So I'd wait until next summer when hopefully covid gets better to do this. If you do want to give it shot though, you can email professors anytime and ask them. See what they say and follow up with them if you cant do it right now.