S.L.
If she is doing well speaking English at school, why do you not want her to use the home language? Be proud that she sees the value in being Trilingual! Allow her to practice and maintain Tamil and Telugu, as long as she practices her English at school.
Also it is much better than she practice English at school with fluent English speakers and does not spend too much time speaking English with any who have a heavy accent or poor grammar. She will pick up the accent and grammar used at home rather than at school.
S. (ESL teacher)