Finding an intercept on percentage based data and using dummy variables
As Jamzy noted, run an OLS regression on grades against whatever variables you have.

$$\text{grades} = \beta_0 + \beta_1 x_1 + \cdots + \beta_i \ \text{race} + \cdots$$

$$\text{race} =\begin{array}{cc}
  \Bigg\{ &amp; 
    \begin{array}{cc}
      0 &amp; mixed \\
      1 &amp; Chinese \\
    \end{array}
\end{array}$$

or vice versa with the 0 and 1. $\beta_0$ will be your intercept that you are looking for. If it ends up negative, try taking the log of both sides and see if a linear regression still has a good fit for you there.