Faulty assumption: there is an advantage to a HS writing her own letter. In my experience, students write lousy letters of recommendation because they do not have experience with the genre and dont have to vocabulary to brag in subtle ways. I’ve never seen one that I could sign without significant work, though less work than starting from scratch.
Faulty assumption: one can spin off meaningful, helpful letters without spending time describing the student’s specific abilities. Really a good letter of recommendation is full of claim AND evidence. Saying "she is exceptionally . . . " works much better if one can write “she has a column in the local paper” or “exhausted the school’s offering in math and science” or “developed an innovative approach to the after school program.” I’m writing a letter in support of a graduate school application tomorrow. I will respond to the draft of the student’s personal statement. Over 1/2 of students don’ t get the format and purpose and need advice. After I have sent her back comments on it, then I will write the letter, going back over the student’s writing for the course and discuss it in the letter. I will incorporate any specifics that support the personal statement. I know this process will take me about an hour even though it’s a straightforward letter. Yes, I could do less more quickly, but I want to support the student’s application.