Yuri promised to her schizophrenia to become a guard.
Yuri did not really promise to become a guard. She consented to ingesting Dar-Phemo regularly and - as a means to that end - she promised to get hugs from him. It's not really necessary for her to be appointed as his escort for that, but claiming a spot in Sou's guard team would definitely allow her to get pretty close to him.
Let's assume Yuri actually plans to become a guard. If I was her, my first step would be to get close to Sou's family. They are the key factor for the most pressing issue at hand:
Sou stops coming to school -> no more hugs -> no DarPhemo -> mission failed
So, going by that, becoming their guide for Class Observation is a must. After that, I would try earning their trust which will be hard enough to do, since they distrust every girl as a potential threat. In the end I would suggest that if they are still worried about Sou's safety, then they should "ask the school to replace the most troublesome guard by someone more suitable"... the question is whether Yuri can fool Sumire's female-in-heat scanner and Sakura's threat sense.