All the above is easier, and you get better results, if you use a tripod to make sure the camera is in exactly the same position each time. This is something you won't be able to do with a mobile phone.
Also, if you're doing this out of doors, you need to be aware that the sky will change.
I know this is a project, and projects are meant to be done mostly at home, but shouldn't the teacher be able to advise on techniques? It's a bit raw to expect a parent to have to help out.