See responses to Len_H further below for more clarity on this, in addition to what Thrillwaters states below.
It wouldn't work because "they were …" requires the "estar" conjugation, "estuvieron," instead of the "ser" conjugation.
The answer to this should not be "they did not go to school today" rather "they were not at school today", thank you
The sentence says 'a la escuela'- to school rather than 'en la escuela'- at school, that is why the verb meaning is 'to go' rather than 'to be'. Looking at the verb in isolation both meanings are possible but the preposition gives it away and only the verb ir makes sense.
That being said, the two sentences do have the same meaning. I'm never quite sure whether that's a good reason to get DuoLingo to accept both...
Although the difference is subtle ( and really mean the same in English) if Duo accepted both we wouldn't have the opportunity to learn the difference. thank you mschwabnz for explaining the distinction. A lingot ro you.
BruceHoltb: No. "van" is present tense. This sentence is past tense; therefore, the past (preterite) of "ir" is "fueron" ("they went") ---- or in this case, "no fueron" = "they did not go".
Also, perhaps more importantly, if the meaning were 'they were not at school' estar would be used instead of ser, because the state of being or not being at school is temporary. ie ellos no estuvieron en la escuela hoy. Fui/fuiste/fue etc. are mostly only used as was/were to describe characteristics (it was nice) or dead people (he was a teacher) - not locations.
The use of articles is hard to comprehend for English speakers. Sometimes Spanish uses articles where English does not and vice versa. It isn't just that "suddenly it's acceptable". The article should be translated when it is correct to do so, and should not be translated when it is incorrect to do so. In this case ("la escuela), the article is used in Spanish but not in English.
If it does not destroy the meaning that is trying to be conveyed then it is probably safe to drop the article in the English. Most likely if "They did not go to the school today" they probably also did not go to another school either.