Translation:I am collecting the children from the school.
The question would be Cé mhéad páiste a bhailigh tú ón scoil? . “Have you collected” is an English past perfect, which is usually translated by an Irish simple past. An English “-ing” form would be needed to keep using the Irish verbal noun, e.g. “How many children have you been collecting from the school?” would be Cé mhéad páiste a raibh tú a bhailiú ón scoil? ; the ag changes to a (which causes lenition of the verbal noun) because the object now precedes the verbal noun.
She definitely said táim.
tá mé is marked incorrect because that's not what she said. Grammatically, táim and tá mé are equivalent, but you were responding to a "Type what you hear" exercise, and if you didn't hear exactly what she said, you fail that type of exercise.
Duolingo doesn't support alternative answers for "Type what you hear" exercises.