Beta particles are a little more penetrating, and gamma rays even more so.
The smoke detector has an americium source, which is an alpha emitter. These particles are easily stopped by a sheet of paper, or by a few cm of air. It's important to emphasize that although radioactivity can be quite dangerous, and exposure should be minimized, we are surrounded by safe levels of low-level radioactivity on a daily basis. In other words, the students shouldn't panic and go home and throw out their smoke detectors.