The stock market is risky. Public funds rarely are placed in risky investments, even if the investment vehicle is one of the best options out there if properly diversified.

Who would run it? Economics teachers in high school do not all understand the market. Some might, but every school does not have investment savvy teachers. A teacher needs a bachelors degree to teach and it does not even have to be in business (or specifically finance). Some teachers might be qualified, but how would you determine if your school had the right staff?

Plus how would the school regulate it? How would they ensure that the money is not placed in risky investments? How could they justify loses in investment value to the government? I have lost money in some years and well I had to only justify it to myself.

Lastly, schools do not tend to have tons of spare cash. They have a budget they need to stick within. They cannot just borrow from the budget to invest. Even if they wanted to, the majority of the school’s money is not available to them till they justify its need.

Universities on the other hand do invest small portions of their capital (some private ones anyways).