Finally, a worthwhile question. So here's what you do...
Get some newspaper, carpenter's glue, a large bowl, water, a piece of PVC pipe, baking soda, vinegar, red food coloring, some paints, and a big piece of wood to use as a base. Now to start off, you have a couple of options. You can make it out of clay and then cover it with paper mache (that's kinda cheating though, your teacher might not like it done that way, plus it'll be really heavy), you can get some chicken wire to shape the volcano and then put paper mache over that (your teacher might like that because it's a bit more advanced, that's how some parade floats are made and such. It's kind of a professional way to do it. Or you can take some of the newspaper, crumble it up into balls, and make most of the volcano like that and cover it with paper mache. If you just did paper mache without some sort of base like I'm describing, it will take you a couple of weeks to do, and it'll require tons of paper, because you would just be putting paper on top of paper, trying to make a big volcano, and well, that's just not how you do it because it wouldn't work right and such. Anyways, moving on. The PVC pipe is to put in the middle, that's the hole where the lava is, so make sure whatever base you choose, you build it around the pipe (and glue the pipe down so it stays, you should pour a little water down it to make sure it's sealed, because you'll be pouring things down it later, and you don't want it leaking out). So once you have the base set up (I highly suggest you get some chicken wire, and use that), you tear the newspaper into long thin strips (not too thin, just a couple inches wide), and pour the glue and water into the big bowl, and mix it up a bit. The point of this is to reduce the viscosity of the glue (basically, make it so it's watery glue, you don't want it thick or else you'll have a difficult time dipping the paper in it). Mix the glue and water well, and then dip the strips of newspaper into the bowl (flip it over so you get both sides) and stick it onto the volcano. You don't need to make it all face one direction or anything, just keep slapping it on all over till it's applied thick and evenly all over the entire thing. From there, grab the hair dryer, and use it to speed along the drying process. You can just wait over night, but this way you'll get it done faster and have more time to do other things. So once it's all dry, start painting it to look like a real volcano (use mostly brown, so it's like dirt, and some red near the very top so it's like lava), and paint it evenly. Hair dryer again so you can hurry up and get this thing done, and then you're mostly done. The rest of this is just extra-credit type stuff. If you do all of this, you'll almost definitely get an A, unless your teacher really hates your guts or something. So now pour the baking soda into the PVC pipe, and a lot of red food coloring. After that, pour the vinegar in, and tada, your volcano will erupt. This isn't a one time thing, but you should wait until you're at school to do it anyways because it doesn't quite look the same after you do it the first time. Now to decorate your volcano, get some grey clay, and roll it into small balls, then shape some of those balls a little weirdly (like make some egg shaped, and some a little flat and such), and stick them to the sides of your volcano (so they're rocks on the mountain side). Now to add some foliage (leaves), you'll want to get some dried moss, you can pick some up at any art supply store, and it looks like this:
It comes in a bag, and you just it straight onto the mountain side. You should make your rocks small so it will look like the moss is like a small grove of trees as opposed to just some piece of moss glued to the side of a paper mache volcano, it helps create the illusion. That's about it, now you have a real professional looking volcano there.
A couple reminders:
1) Assuming you're a kid, you should do this with parental supervision.
2) Be careful, chicken wire is sharp.
3) You should bend the chicken wire a little, mountain sides aren't completely flat.
4) Don't make the rocks too big.
5) Stick the rocks down well or they'll fall off, you may even want to dry them out with the hair dryer and then glue them on with super glue (have your parents use the super glue).
6) Make sure that the PVC pipe is really glued down solid, make sure it's properly sealed with a little water. I know I said this before, but you really really don't want it to leak out the bottom or else it won't have enough pressure to spill out the top.
7) Don't forget the red food coloring, lava isn't white you know.
8) As soon as you finish the paper mache part, before you dry it with the hair dryer, wash your hands and the bowl well, because it's difficult to clean once the glue dries.
9) Before you start all of this, you should put some extra news paper down on whatever surface you're working on, that way if you drip, it won't be an issue. If you're going to go with news paper, do two layers, so it doesn't leak through.
And one final tip to finish it all off... If you get some cotton balls, and tear them apart, you can have your parents super glue them to the top of the volcano rim, so it's like smoke billowing out of the volcano.