Murals in the classroom
I am not a teacher, but I too am impressed with your willingness to commit.. Do not take this as permissive, but my belief is that your education begins with your interests and the rest can be garnered in college, where you can apply yourself. 

If you intend to be an artist, then this is your launching pad.

Time is always a problem when stepping into a new endeavor.  Please consider using a grid and transferring the entire image, in pencil or chalk, onto your surface.

I suspect you can google all you'll need to know about using a grid for reproducing an image onto a wall, so I won't elaborate to much here.  


Sketch your idea or use a commercial photo.  
Then overlay clear velum, glass, plastic, or draw a grid pattern directly onto the sketch.
With a chalk snap line, make a larger grid on the wall.
Now, imagine each square on your sketch or photo being copied onto the corresponding chalk line square on the wall, then draw it.


This will save you an enormous amount of time in every step of the mural process.  Designing, transferring, and painting can all be done on your sketch, and preparation, the key to success, is covered.