I copied this from an English grammar forum: "A search in Corpus Of Contemporary American English for "I teach ... school" (with a gap of 0 to 4 words) yields 44 results, which can be classified as follows. This corpus has five subcategories: spoken (SPOK), fiction (FIC), news (NEWS), magazine (MAG), and academic (ACAD).<pre>
a specific school (16 hits) The possible constructions are at the, at a, at [Ø] (zero article), and in a, as illustrated below: I teach at the school my kids attend. (MAG) I teach at The Farm Cooking School. (MAG) I teach at a school with a high number of' gifted' and high-achieving students. (ACAD) I teach Arabic at a nearby school (FIC) I teach in a school with hardworking, knowledgeable teachers. (NEWS) I teach in a private school. (NEWS) I teach students at [Ø] Columbine High School. (SPOK) school with general meaning (15 hits) The possible constructions are apparent from the examples below: I teach summer school every year. (ACAD) I teach school. (MAG (1 time), SPOK (3 times)) I'm a Roman Catholic and I teach Sunday school. (SPOK) I teach high school here at a private school. (SPOK) I teach history in high school. (SPOK) I teach religion in high school. (SPOK)"</pre>
I suppose that "in that school" is possible too, but requires a Dutch equivalent of in die school, i.e. in the actual school building instead of at the abstract learning institution called "school".
I typed in My Mother teaches at that school. And was told that it was incorrect when I repeated adding My mom teaches at that school that was also incorrect I even tried typing both with a comma after the first full stop and got the same error. To my way of thinking I should have received a correct answer for my first answer,
My mother teaches at that school., My mom teaches at that school.
This is the answer registered in your report. It will see that as one full sentence. Not two separate.