Deepti went to school for 216 days in a full year.

Let the school opened for $x$ days in the given year.

We have :

$90 \%$ of $x=216$

$\Rightarrow \frac{90}{100} x=216$

$\Rightarrow x=\frac{216 \times 100}{90}$

$=240$

$\therefore$ The school opened for 240 days in the given year.