There's no single, readily available number for the exact total number of students attending school in the US. The count varies depending on what levels of education are included (pre-K, K-12, higher education) and the time of year. Data is collected by different agencies, and compiling a perfectly accurate, up-to-the-minute total is a complex task.
To get an estimate, you would need to consult data from sources like the National Center for Education Statistics (NCES) and combine data from various reports covering different educational levels.