For more ideas, check out (some of) the 300 thousand projects that have been published to scratch.mit.edu.  Do not simply copy someone else's work of course, or take someone else's work, make a few changes and then pretend it was all your own work. You must be very clear and honest about what you used from other people and what is your own work. This does not mean that it is forbidden to build on someone else's work, just that you must indicate clearly what YOU added.
 

Also check out the video tutorials at learnscratch.org. I don't guarantee that these videos will be accessible form school. The first tutorials are much too easy but some later ones are well worth checking out, such as:

You can also check out www.redware.com/scratch for videos, tutorials and ideas.
 

Look at this page of suggestions for programs that could be made with Visual Basic - most of these programs can also be made in Scratch.

Although Scratch can import JPEG, GIF, PNG and BMP image formats the only format that imports without any colour distortion is BMP, so if you are going to import an image and you are a perfectionist like me then prefer the BMP format. For example, if you have a GIF image you want to import then open it first in an image editor such as Paint Shop Pro, save it from there in the BMP format and then import the BMP into Scratch.

Whenever possible, use photographic images in your projects both for the background and for the sprites.

Pay special attention to sound effects and music for good music can make all the difference between a mediocre program and an entertaining one.

'Localise' your project: include locally-taken photographs (school, friends, pets etc) in your projects. Not only will this make your programs more entertaining to those who know you but also it will be more obvious that the work is indeed your own.