。。
That's the warning bell. The students around the school gate all picked up their pace at once.
Literal
Warning-bell is. School-gate [gen-の] vicinity [at-に] was students [topic-は] all-together foot [obj-を] speed-up.
校門 ('school gate') is the front gate of a school, a social gathering point in Japanese student life. 一斉に ('all at once, in unison') is a formal adverb, commonly used for coordinated group actions. 足を速める ('to speed up one's steps / pace') is the transitive form of 速まる — a common collocation for hurrying. 予鈴 ('warning bell') is the pre-class bell that rings a few minutes before the main 本鈴 ('main bell') — Japanese schools traditionally use this two-bell system to give students a buffer to hurry to their seats. The sentence is a compressed scene: four clauses in just a few words.