I would follow this guide. Gives you the exact stat and skill rank required to recruit the students. Someone like Felix requires B+ rank in sword and a 15 speed stat, and Lysithea actually requires a B rank in Faith and a 15 magic stat

Included is a table showing how much support levels lower the requirements based on the initial requirements. Getting A rank (only doable in a NG+ file) automatically allows you to recruit a student. So Felix with C support now requires B rank in Sword and a 12 speed stat, and Lysithea with C support requires C+ in Faith and a 12 magic stat.

Oh, and Sylvain automatically defects to Black Eagles or Golden Deer if you choose to play as Female Byleth.

Very useful for my Blue Lions play through cause I don't want to kill my Golden Deer babies.