If an intact loop has caught on the outside of a knitted school jumper, guide it to the inside with a small hook. Leave the loop attached. Cutting it can turn one snag into two loose ends that are harder to secure.
Check that it is still a loop
Lay the jumper flat and find where the yarn emerges and goes back into the knit. An intact loop has no free end and the stitches around it are still linked. A hole, a broken end or a vertical ladder of open stitches needs a different repair. If the snag is at an embroidered logo, first check whether you are looking at the embroidery thread rather than the garment yarn.
Bring it through at the same point
Use a small crochet hook or a snag repair tool that fits between the stitches without stretching the knit. Work from the inside and pass the hook through near the base of the loop. Catch the loop and draw it through gently. If it catches, back off and take a smaller bite. Avoid pulling from a different row, which can leave a diagonal line across the face of the jumper.
Ease out a pucker
Hold the garment flat and gently ease the nearby stitches in the direction of the knit. If the snag borrowed yarn from the surrounding row, this can spread the slack over several stitches. Work a little at a time and stop when the surface lies flat. Leave any remaining loop on the inside where it will not catch during wear. A long loop in a high-friction spot may need securing by someone familiar with that knit.
Recheck the outside in daylight
Smooth the jumper with your hand and look for a ridge or a line of tightened stitches. There should be no new gap. If you find a broken yarn or a small hole with stable edges, move on to the darning guide. If stitches are continuing to run, keep the garment flat and get the loops secured before washing or wearing it.