I’ve run roleplaying at my school 
for 17 years (pupils aged 10-18), plus an occasional game for friend’s families.
I use 1st edition AD+D together with a printed extension volume created by my adult group (which has been running since 1979 
)
I echo what posters have said already:
- start with simple characters, mainly fighters and perhaps a healing cleric
- keep the stories something they can relate to, with strong good v evil plotlines
- keep the dice rolls simple and tell them what they need
When visiting other schools with a demonstration game with complete beginners, I used the scenario below. I can probably dig out the files (which include all the dice rolls for new players) if you’d like. Feel free to e-mail me.
Glee’s intro dungeon
The player characters are:
Crumlin (1st level Cleric) - played by the Dungeonmaster
Paladin (3rd level) - female version available
Ranger (4th level)
Dwarf Cleric (4th level)
Druid (4th level)
Magic-User (4th level) - female version available
Hafling Scout (5th level)
Game introduction:
- the Paladin could have a mock battle with the Ranger
- the Cleric and Druid could heal the fighters (and learn to use First Aid), then see who has a magic item amongst the party, or if anyone is Charmed
- the MU and the Scout could try sneaking past a ‘guard’, getting over a wall, and first finding, then removing a trap.
Storyline:
Crumlin is standing for Mayor of Haranshire, a countryside province. His rival is Dag, a nightwatchman, who is clearly a simple stooge for the rich landowner Darius Kharman. Kharman is ruthless and greedy. His attitude to the election is ‘One man, one vote…I am the man and I will have the vote.’
Crumlin wants the other player characters to help him fight the election fairly. Kharman will undoubtedly try to use dirty tricks, and the party have to find ways to stop him!
Background:
Haranshire is a rural area, and Milbourne is the County town. There is only one other village, Harleton. The principal activities are farming, fishing, forestry and mining.
The chief landowner is Darius Kharman. He also owns all the mines.
Most of the population is human or dwarvish. There are also some haflings and a lizardman community living near Harleton village.
The election will be supervised by Laff, a High Priest, who looks after all of Haranshire. Crumlin knows Laff is honest, but a little gullible. (Laff thinks the best of everyone, even Kharman).