。
She's set to give a speech on behalf of the class.
Literal
She [topic-は] class [object-を] representing speech-give [it-has-become-the-arrangement-that-ことになっている].
Two grammar layers. ~を代表して ('representing X, on behalf of X') is the te-form of 代表する ('represent') functioning adverbially — the speech is delivered in a representative capacity. ~ことになっている ('it has been arranged that, it is established that') describes a settled arrangement, rule, or expectation: someone — possibly the speaker, possibly an external authority — has decided this will happen. Compare to ~ことにする ('decide to do X' — the speaker's choice) and ~ことになる ('it turns out that' — emergent result). The ~ている form locks the arrangement into the present as a still-standing plan or duty.