Because of the "on Sunday" part, I think that makes "am going" the simple future.
How do you think you would you say, in Spanish, "I am not going to school on Sunday"?
I tried it in Google Translate, and it came up with "Yo no voy a la escuela el domingo" (i.e. pretty much the sentence from this example).
Maybe there is an implied "to go" in there, as in "I am not going (to go) to school on Sunday", in which case the Spanish would be something like "No voy a ir a la escuela el Domingo".
But c.s is wrong even with that correction I'm afraid.... Voy DOES mean I am going as well as I go and I do go (typically used for questions and negatives). The estoy yendo construction is NOT equivalent to the general use of I am going to school (eg I am going to school now that I am five, I am going to school until summer...) which is translated to Spanish by "voy". That construction, though it seems to resemble the English one, is used specifically to mean I am going to school right now, eg I can't chat on thge phone now because I am going to school [undesrtood: at this moment]
It would be pretty close a translation to "on" instead of "the." Only use "los" if you go EVERY Sunday. Like, "Voy a la escuela el domingo" means "I'm going to school on Sunday." However, "Voy a la escuela los domingos" means "I go to school on Sundays." And then este means "this;" "esta" before feminine nouns.
rspreng answered this above and manofsnow has added even more info. The summary is that "el", not "en", is used before days of the week in Spanish. If you didn't read the comments before posting, please try to make that a habit. If you did, please try to read more carefully, or at least do a "find in page" search.