。
She's very popular among the students.
Literal
She [topic-は] students [possessive-の] among [at-で] very popular-existing.
学生の間で is a stock phrase for 'among the students,' with 間 ('interval / space between') used to mark social space — the figurative gap among a group of people. で marks the location/scope of the popularity. 人気がある ('be popular,' literally 'have popularity') uses 人気 as a noun meaning 'public favor / popularity,' and the existential ある to predicate possession. たいへん is a slightly formal-feeling intensifier ('very, greatly') — more written-leaning than the everyday とても. The whole sentence reads as the kind of description a faculty member or alumnus might give of a beloved teacher.