Sapper School

Not surprised. Sapper school crams a ton of stuff into a short period of time. Demo calculations and the multiple steps required in the different reconnaissance elements are a royal pain on sleep deprivation and patrolling is also tough.
Before the Tab was authorized whole units (platoon sized elements) went through together and passing was a crap shoot based on the skills of the units leaders. Since the Tab became authorized the course is open to nearly all MOSs like Ranger school. So folks who are not Engineers tend to have trouble with all the engineer reconnaissance missions and target folders for , bridge, route, demo, etc (ain't just kickin' doors). The physical requirements are also brutal, these cats cover some ground during the course.
While I never earned the Tab, I did serve as the CSM for the battalion and later the Brigade that runs the course and had the opportunity to participate in many of the training events. The instructors are the best the Engineers have to offer (they are/ were screened by the Regimental CSM) and they will smoke your bag. So both suppositions Andrew puts forth are quite probably true.

I went to sapper leadership school at ft leonardwood mo and it was a challenge they did not have a sapper tab back then and it was a brutal 4 week course. It was a team environment and believe me you needed your team.

I'm quite interested to find out the rest of this story.