Cohomology of unipotent group schemes

We verify that universal classes in the cohomology of $GL_N$ determine explicit cohomology classes of Frobenius kernels $G_{(r)}$ of various linear algebraic groups $G$ . We consider the relationship of $\varprojlim_r H^*(U_{(r)},k)$ to the rational cohomology $H^*(U,k)$ of many unipotent algebraic groups $U$. The second half of this paper investigates in detail the cohomology of Frobenius kernels $(U_3)_{(r)}$ of the Heisenberg group $U_3 \subset GL_3$.