Examboost, the idea of two teachers from Uppingham school in Rutland, is backed by Internet service provider Totalise. David Lewis, head of biology, and head of physics Martin Roberts, said they wanted to encourage pupils to work harder.
"Examboost encourages pupils to achieve the best possible grades in their exams and educates them about share ownership," they said. Pupils only need to sign up for a free Totalise e-mail address to participate.
The number of shares they receive depends on their grades in three subjects - an A at A-level earns 40 shares, with 32 for a B, 24 for a C,16 for a D and eight for E.
At GCSE, only English, maths and science are considered: an A* nets 20 shares, an A 16, a B is 12 and a C gets eight. Shares will be held for students until 18.
The value of the shares depends on Totalise's worth on Ofex, a London exchange that lists securities of companies not quoted on stock markets. This week the share price was 62.5p, making three A grade A-levels worth pound;75.
However, Chris Thatcher, president of the National Association of Head Teachers and chair of its IT committee, said: "I am concerned about this because I want to know that it is in the best interests of youngsters and I am not sure it is."