Jenkins is on the money on this one.

Umpiring as an IC is is a slippery slope that has yet to be set in federal law with eight states having IC/Officiating laws, and six states having IC/Officiating "rulings":
http://www.naso.org/rprt/SpecReptIndCont.pdf
The NASO article also agrees with Jenkins and I, that someone umpiring for their employer means employee when umpiring.

A person working for a school district would be an employee/official if officiating at any school within the district.

There is no harm for the employer if they classify someone as "an employee for one item, an employee for all", and a huge risk if they try to play the employee sometimes, IC other times "game".

The following article discusses a school setting from another perspective.
Higher Markets’ Purchasing Pulse newsletter in September, 2000
http://www.rainassoc.com/employee.pdf

"Of over thirty cases in which the US Tax Court, US District Courts, the US Courts of Appeals and the US Supreme Court have issued opinions based upon Revenue Rule 87-41, there is not a single case where a taxpayer has successfully argued that a person can be an employee and a contractor concurrently for the same employer."

"And as if to add insult to injury, in some instances 26 USC 6672 gives the IRS authority to hold "responsible parties" liable for a penalty equal to 100% of the employment taxes not properly collected and paid for employees. A "responsible party" can include officers, managers and any other person who exercises a controlling decision in the process of whether or not to collect payroll taxes — and that means you and me folks. There’s no question that there is some degree of personal liability as a consequence of improperly classifying employees as independent contractors.
So here’s the denouement — if your institution is issuing an IRS Form 1099-MISC and a W-2 to the same name, you just might be announcing that you’re underpaying the tax man and you probably should prepare the welcome mat for the IRS auditors."

I used to work for a school district at the high school building and did freshman and sophomore games for them, as well as a few games at the middle school. I did not do varsity games at the high school. The pay for those games was always put on my regular paycheck. I always felt that if I was ever injured doing one of those games, it would be considered the same as getting injured while doing my regular job for the district since I was not being paid as an independent contractor.

bingo!

Employee for payroll withholding purposes and employee for WC are cheese and chalk. WC is a state issue, for one; IC/employee an IRS one. They are not necessarily mutually inclusive.

IC status relieves the customer of mandatory payroll tax withholding obligations--they don't have to crate a new subsidiary account for the IC. If you're already an employee, that's a non-issue and one that school districts don't want to argue: "Why didn't you withhold based on ALL his compensation?" Hence, your friend's predicament.

Officiating is (probably) not part of the teacher's official duties; he can decline any assignment working for his employer's teams without recourse. He can say, "I'm not working our school teams' games", but (probably) can't say, "I'm not teaching third-period English."

The variance in state WC laws precludes a definitive answer here. In my state, he'd have a weak case for a WC claim based on an injury sustained while officiating. He wasn't performing his "official" duties (pun intended), and while he is being compensated by the school, he isn't "at work."

It may be different in your state. There is no one dispositive rule that does or does not make one an IC, and simply having taxes deducted from your check doesn't automatically make one an employee.