* Size of the high school: High schools vary drastically in size. A small rural high school will have far less space than a large urban one.
* Layout of the high school: The number of classrooms, hallways, and open spaces will affect capacity. A school with a lot of open areas could fit more sheep than one with mostly small, crowded rooms.
* Size of the sheep: Sheep breeds vary in size. Larger breeds will reduce the number you can fit.
* Sheep density: How tightly you pack the sheep will significantly impact the number you can fit. Ethically, you wouldn't want to pack them too tightly, as it would be inhumane.
To even get a rough estimate, you'd need to know the cubic footage of the high school and the average volume of a sheep. Then, you'd need to make assumptions about how tightly you could pack them without causing harm. This would still be a very rough approximation.
It's safe to say it would be a very large number, likely in the hundreds, possibly even thousands, but providing a precise number is impossible without far more specific information.