Lots of bugs on PE

In my school we have a ipad program which mean I always have minecraft and on the free time I play always.
One of the issues is worlds not saving.. I'm not sure if it is because someone is in your game with you or what? But I have packs of iron and some diamond

Another is when I'm in someone else's game if I open a single chest the minecraft app will instantly crash.

Also there is one with gravity.. When you happen to place sand like 3 up so it will fall it disappears, there even isn't a block.

Also there is one that when my friend joins the game It won't say it on my screen then it will then they will leave the game. This happens a lot