en camino = you are going to somewhere.
de camino = while you are going you will do something
Voy en camino a la escuela = I'm going to school.
De camino a la escuela te llamo = while I'm going to school I'll call you.
Good explanation, but the firt example is wrong, you must say: "Voy camino de la escuela", without "en"
"En camino" is used with the verb "estar", because it shows the place where you are (in this case on your way).
Estoy en camino = I'm on my way