Terrain glitch

I have been making a world on creative mode which I have been flattening out. What I did was place grass blocks above the terrain (which took me about 2 months to finish). I was planning to make a city server on this map. When my friends join me on it they can see random bits of terrain sticking up over the grass blocks which I can't see and can go through. I have jailbroken my ipad and after I did that it's when it started. If you could help me I would be really thankful! Bye!