if you are using coke and mentos , it has to be diet because asparetene (sp) is needed to create the reaction.
we made one in class with some of our mid range students ( academically they are probably between 10-14)
ours was made with a 500ml bottle of diet coke as the centre.
over the top was a cone shape made from thin cardboard
this was covered with a thin layer of pva glue and cheap crepe bandages ( a box from poundland, cut into smaller strips). this gave a uneven texture mininal effort. that was then painted with brown paint, with slightly different shades of brown . ( brown, brown with a bit of white mixed, brown and black, brown and grey etc)
once dry, place cone over bottle. mark about an inch down from the neck of the bottle, and cut vertically across the cone, giving you two pieces, one bigger than the other
hinge the two pieces on one side with a bit of duct tape
if you want a cross section of a real inside volcano, then use the cone as a template, and draw the cross section on a bit of card. stick to side of coke bottle
put bottle of diet coke on a plastic plate, and put cone on top
secure cone sides with a split pin on each side, and a loop of string or wire
varnish outside and inside of volcano
put some scrunched up red paper in the top to hide the cola lid
when you are ready to erupt your volcano, get a piece of tubing slightly bigger than a mentos ( we used a smarties tube) take paper and lid of the the cola. put a piece of paper or card over the neck, put tube on top, fill with mentos, the remove cars, dropping mentos in to bottle , and voila
if you dont want a mentos style erruption, instead of mentos, you can fill an empty bottle with a mixture of gelatine and red food colouring, or even cheap liquid soap, a bit of water and food colouring, put a bit of wire wool with talcum powder sprinkled on top of it and put a drinking straw in between the wire and bottle , and just blow when you want a ash cloud and bubbly larva
