How many students in the school? 
If they put the rows of 13, there are 8 students left; if they put the rows of 15, there are 3 students left and if they put the rows of 17, there are 9 students left. How many students are there given that the total students are < 5000?

The following congruences are: $$x\equiv 8 \pmod {13} \\ x\equiv 3 \pmod {15} \\ x\equiv 9 \pmod {17}$$
I'm still new to this, how can I apply Chinese Remainder Theorem to these congruences to find the total students?

It's easy since here since the moduli are in A.P. (Arithmetic Progression) $\,13,15,17\,$ so we can apply a simple inversion-free version of CRT that only needs to invert the A.P. step-size (here $2$). First we transform our residues $\,8,3,9\,$ to $\rm\color{#0a0}{con}\color{#90f}{gruent}$ ones in A.P (using the Remark below), so
$$\begin{align}
&x\equiv 8\equiv\ \ \:\!  \color{#90f}{99}\!\!\!\pmod{\!13}\\
&x\equiv 3\equiv\ \ \  \ \:\! 3\!\!\!\pmod{\!15}\\
&x\equiv 9\equiv\! \color{#0a0}{-93}\!\!\!\pmod{\!17}_{\phantom{|_{|_|}}}\end{align}\qquad\qquad$$
thus $\!\bmod \color{#c00}{13\!+\!2n}\!:\ x\equiv 99\!-\!96n\!$ $\iff\! 2x\equiv 198\!-\!96\smash[t]{\overbrace{(\color{#c00}{-13})}^{\large \color{#c00}{2n}}}\equiv 1446\,\smash{\overset{\large \div\,2_{\phantom|}\!}\iff}\, x\equiv\,\bbox[5px,border:1px solid #c00]{723}$
Remark $ $ To get residues in A.P. we solve
$\,3\!-\!(8\!+\!13j) = (9\!-\!17k)\!-\!3 \!\iff\! 17k\!-\!13j\equiv 11$ $ \!\iff\!\!\bmod 13\!:\, 4k^{\phantom{|^|}}\!\!\!\equiv\! 11\!\equiv\! 24\!\iff\! k\equiv 6\,$ so $\,j = 7,\,$ so $\,8\!+\!13j=\color{#90f}{99},\,$ $9\!-\!17k=\color{#0a0}{-93}$
