On the first day, the student can choose any $1$ of the available $6$ kinds of sandwiches, so total number of ways to choose a sandwich on first day will be $^6C_1=6$. Similarly, the student will have $6$ choices on each of the next $6$ days, hence total number of ways to choose sandwiches for the $7$ days of week will be:

$6*6*6*6*6*6*6$

$=6^7$

$=279936$