If a marimba or xylophone bar is out of tune it is not difficult to restore it to the correct pitch. Chiselling or rasping (or even sawing) under the middle of a tone-bar will deepen its pitch. Removing wood from the ends will raise the pitch. You can saw wood off the ends but your bar will look wrong on the marimba!
If you are concerned with tuning for harmonics, the process is more complicated. A technique for doing this is explained in 'Make your own Marimbas'. For most school instruments however, the simple system described above is fine. A chromatic tuner is very useful for checking the tuning of bars. Small chromatic tuners are now relatively inexpensive.
If the bars are cracked or split, they will need to be replaced. If your marimba has bars suspended on rope, make sure the rope is not sitting on the wood frame. Tighten the rope if necessary.
You can check the tuning of a pipe resonator by tapping on the cap while holding a chromatic tuner near the open end of the pipe.