Mary is strong in/at/on Maths.

In the UK we would usually use at, occasionally we might say in, but we would never say on.

Same in the US.

Prepositions are mostly interchangeable. Some languages don't even have a selection. For example, in English we think there is a difference between a knife on the table and a knife in the table, but Spanish makes no distinction; the knife is in the table and that is the only way to say it.