Why do I get erect so easily?

Sometimes I'll just be walking around and I'll get one, or maybe in the middle of class. The worst is if I'm just like walking hand in hand with my girlfriend. I'm not even thinking about anything sexual, it just happens. I know that one reason is that I am 16, but is there any way to control it? (besides thinking of old people sex)

You're at the age where you will often get erections for no reason that you can tell. At some point during puberty, most boys go through a period where their body decides their penis needs a regular exercise program. When your body decides it's exercise time, it doesn't care whether it's going to be inconvenient or embarrassing for you, it just goes ahead with the exercise. It doesn't matter what you think about, because thinking is not what caused this in the first place. You just have to move your penis to some arrangement where it's as comfortable as you can manage, and wait for it to go away.

I remember this used to happen to me like clockwork every day in the middle of algebra class, which left me bewildered because I wasn't turned on by algebra or the algebra teacher.

Try to look on this in a positive light. You're brimming over with health and vitality, and that shows in rock-hard erections that sometimes occur all by themselves. In the future, you'll probably look back with some wistfulness to this time.