Job application question

I have an interview coming up so they already have my resume but I need to fill out their application and turn it in at the interview. Of course the application includes a line for 'reason for leaving' that I need to fill out for all of my past jobs. So my current one is easy (leave replacement)...but what do I put for the rest? I always try to get away with 'to begin X position with Y company' but is that cheating?

I left the first school I was at in the middle of the year to take my current leave replacement position, so what the heck do I put there as my reason for leaving..
'hated administration'?
(note, the job I am interviewing for is NOT in a school).

I'm realizing that I've worked now in 4 different places since graduating about 7 years ago...and am concerned about minimizing that so any advice would be greatly appreciated

I would word it like 'Left position to take new position to....'
and then put in the reason, better job experience, developing a particular skill, etc. etc.