It is common for tidal volumes to be slightly elevated when using a spirometer in the classroom. Any subject with a foreign object in their mouth will reflexively breathe deeper than normal. If you are using disposable mouthpieces, recorded tidal volumes of 0.8 to 1L are not uncommon.
Naturally fitting mouthpieces tend to give better results. They make the subject more comfortable, so the subject breathes in a way that is more normal. As a result, tidal volumes will be in a more normal range (0.5-0.8L).
Naturally fitting mouthpieces also do not leak like the disposable ones, so the data is also more accurate.
More information about naturally fitting mouthpieces can be found at Do you sell mouthpieces for the Spirometer that are reusable or autoclavable?