Charging capacitors with computer power supply?
A computer power supply is really not suitable for charging photo flash capacitors to near working voltage. It may be safe, if you use a series resistor to limit the current and take care as to the polarity(!), but it won't charge them to anywhere near full voltage, and since energy is proportional to voltage squared, the energy storage will be very low. 

Charging photo flash capacitors to near working voltage, particularly in parallel or series groups, is not a suitable project safety-wise for high school students, particularly unsupervised, no matter what some of us may or may not have survived in our youth.