'I was told that there would be no maths.' Surely, this is perfectly acceptable English?
British variants are normally accepted automatically by Duolingo. If this one is not, they may have missed it.
Thanks, I thought that was the probable explanation. I did report it.
Fwiw, "math/maths/mathematics" was my standard for all of these sentences from the start, without any reliance on the auto-substitution.