one school
Refers to a single school as an institution. Often used in contexts comparing or counting schools.
On average, there are about 500 students per school.
There is only one junior high school in this area.
the whole school; all students and staff
Used when referring to the entire school community, such as in events or announcements.
The whole school worked together to prepare for the sports day.
first proof; one round of proofreading
Publishing term for the initial proofreading pass. Also used more generally for a single proofreading check.
Once the first proof of the manuscript is done, we'll move on to the second proof right away.
The whole school pulled together and made the festival a success.
The publisher issued the first proof of this book last week.
全校 is the everyday word for the entire school (all students and staff), while 一校 can carry the same meaning but sounds slightly more formal or literary.
初校 unambiguously means the first proof in publishing, whereas 一校 can also mean 'one school,' so context decides which sense is meant.
Compound of 一 (one) and 校 (school/proofreading). The proofreading sense derives from the counting suffix 校 used for proofs.