I can relate to that feeling. My DS is 16 and in 11'th grade this year, I have no idea how thats possible already!
Songs that I loved in high school are now played on the oldies channel, my kids roll their eyes when DH and I talk about growing up in the days before cell phones and Internet access. I find myself experiencing the age gap having to explain things my kids have never heard of like 8 track players, pay phones, and basically any reference to the 1980's.
I almost broke out in a cold sweat the other day when my DS mentioned that next Summer will be his last Summer in High school!
Time truly does speed up as you get older!
I hope this is a great school year for you!