The zip in your long boot has tiny, intricate plastic teeth and is inevitably going to be more fragile than the rest of a tough leather boot - treat it harshly and you will break it.
Always pull the zip down completely before putting your foot in or taking it out of the boot - this is important and is the number one cause of broken zips!
Never force a zip closed - if it's jamming, take your foot out and try again.
Keep the zip clean. Remove dirt, grit and school surface with a small brush (eg old toothbrush) and water. Dirty zips will not do up smoothly and you'll be tempted to force it - causing damage.