If something good is going to be thrown away, but if you take
them it might be unethical....then take the donation
approach....
Donate it to needy children.
Next time can consider this.
True
You can offer to bring those new usable stuff to needy kids.
Maybe to those in your sister's class.
Either that or you can ask to salvage some items during the
year's end when the school aunties are bringing them out to the
trash.
No harm taking what's going into the trash. But it's definitely
not ok to take them before the holding period is up.