"I am going to school." can mean that you are literally in the middle of going there right now, that you are enrolled in college/school right now, or that you are going to go there in the future depending on context in American English. Let me demonstrate with some examples. :)
Example 1:
Mark calls Steve on his cellphone.
Mark: Hey, what are you doing right now?
Steve: I am going to school. / I am on my way to school.
Mark: Oh, darn! I was going to see if you wanted to go see the new X-Men movie with me.
Steve: Sorry. I can't, but maybe after I get out.
Example 2:
Mark: Hey. I haven't seen you in forever. What are you doing now? Are you working?
Steve: No. I don't have a job right now. I moved back home and I am going to school.
Mark: Well hey, that is a plus. What college / school are you going to?
Steve: The University of Massachusetts in Amherst, Massachusetts.
Mark: Nice! I went there too.
Example 3:
Mark: What are you doing tomorrow?
Steve: I'm going to school and after that I'm most likely studying at the library.
Mark: Crap! I needed someone to give me a ride to work in the morning. I'll ask Tim then. He might be able to.