It should be right. Report it to DL. This lesson is still in beta anyway

That'd be right. However the correct writing should be "di sekolah" (with space). You should distinguish between the word "di" (meaning at), and the prefix "di-" (which is used to transform verb; for example "di-" + "makan" (eat) = dimakan (eaten).

This, of course, only a problem in written form of the language

"Ada" is a word to describe existence. In English it may be equivalent to "there is/are" or "available". Here's some example of its use :

1. Ada banyak guru di sekolah. Lit. translation : Exist. A lot. Teacher. At school (There are a lot of teachers at school)
2. Bukunya ada dimana? Lit. translation : The book. Exist. Where? (Where is the book?)
3. Masih ada satu kursi. Lit translation : Still. Exist/available. One. Chair/Seat. (There is one chair still available)

Think of this sentence as "you exist at school". Not a very common English sentence, but remembering it this way made it easier for me