I'm in year 10 too. Infact, my schools been on the news pretty much worldwide recently (within the last year) due to Ofsted etc. ("Worst in Carlisle"? Pfft! I doubt it...). Anyway, I had times where we were outside the class for a full hour. Surprising enough, it never, ever affected my Straight A Grades. The problem for me? The lower classes who were in riot and anyone who refused to learn because "it's a bad school". For me, that took up valuable time that could've been better spent learning.
Long story short, no-one likes it when the teacher never shows but, in my opinion, I find it rather minor and rather major at the same time. At the time, I go full-scale panic almost (I don't express this panic, but I do worry), however, it never seems to cause a drastic effect on me - as my grades always remain either static or improving.