To sufficiently charge the iPad, students and teachers should be using the larger, brick-style charger rather than the smaller plug-style charger.
This wall adapter is for smaller iOS devices, like the iPhone or any iPod model.
This larger adapter was introduced for the iPad specifically.
The iPad's larger battery requires more energy to be provided by the wall adapter, so the larger brick is required to fully charge. The smaller charger will work and the iPad will show power in the battery, but it is not at full capacity and will drain quicker with use.
You can print a copy of this diagram from this PDF.