Zack and Mikey sneak into school two hours early to trade on the stock market. They could not have traded since the opening bell of the NYSE is at 9.30am. After hours trading for individual investors did not exist until the late 1990s.
Stocks go up and down all the time. There is no reason that a plummeting stock would cost Miss Bliss her new car. Even if the stock goes to zero, she does not owe any money, she would just lose her investment.
It is unlikely that a school district would allow a teacher to have the students invest their money into actual stocks as opposed to investing pretend money into fictional stocks.