If it rains tomorrow, there won't be a field trip.

Literal

Tomorrow rain [subject-が] fall-if, excursion [topic-は] exist-not.

遠足 ('excursion, field trip') is a school vocabulary word. ありません is the polite negative of ある. The は on 遠足 is contrastive — 'as for the field trip (specifically), it won't happen.'