I would feel awkward about it as a student- they waived their FERPA rights and aren’t expecting to see this. Plus, if it’s a glowing letter would there be a sense of obligation (e.g. I need to always show gratitude to this teacher) for the student that would carry throughout the year…?
If they feel like they need to say something that isn’t “no,” perhaps ask the teacher to save it for them for after graduation.