Um, Teacher? What you wrote on the board is a trigonometric function, not an exponential function...
Literal
Um, teacher? Board [to-に] written [nominalizer-の], exponential-function [copula-ではなくて] trigonometric-function [copula-ですけど]...
指数関数 'exponential function' and 三角関数 'trigonometric function' are math-specific compounds. あのー is the polite, hesitant attention-getter. ですけど... trails off to soften the correction — a classic Japanese strategy for pointing out errors politely without being direct. A very natural classroom interaction.