Yes, Én egy nagy iskolát látok is fine. You can report it if it comes up again for you.
In some cases (mostly with things that occur in bulk quantities) you can use egy to specifically indicate ONE thing as opposed to a mass quantity of it when no article is used at all.
Like if I'm at the grocery store, then
Almát és körtét látok almost certainly means "I see apples and pears"
Egy almát és egy körtét látok carefully specifies that I see one apple and one pear.
Since large schools are not usually seen in mass quantities, I think the sentence in the exercise would be understood in most circumstances with exactly the same meaning whether the egy is present or not.
Does this have a similar sense to "I'm looking at a big school"?- imagine you're on your mobile, describing what you see in front of you. You wouldn't normally say "see", although you might say " I can see". In English "look at" tends to have a little bit more the idea of focussing on something-a bit more active, whereas "see" just seems to be about everything that your eyes are taking in actively or passively. Is the difference between lát and néz similar?