"They will wait at school."
Translation:Arhosan nhw yn yr ysgol.
The English translation should really be 'They will wait in school' with 'They will wait in the school' and 'They will wait at school' as acceptable alternatives.
Thanks for the comment, I've now altered the English translations to the above.
The sense of the sentence is waiting inside the school building/grounds so 'at school' makes sense but 'in school' is closer to the literal translation.
'ar yr ysgol' means 'on the school' and wouldn't be a translation of 'at school'.
There is no equivalent preposition to 'at' in Welsh for this sentence.