Short answer: many charity shops do accept both, but specialist bridal charities and school uniform schemes often make better use of them than a general rail.
Several major charities, including Oxfam, explicitly welcome wedding dress donations — some run dedicated bridal shops or sections since these need different display and storage than everyday clothing. If your local shop doesn't have space for one, a specialist bridal-wear charity shop is usually a short search away and will likely get better value for the dress than a general rail would.
This is where a general charity shop often isn't the best fit, even though many will take it. School uniform is usually crested or branded to one specific school, which limits resale to families at that same school. Many areas have dedicated uniform swap schemes, often run through a school's own PTA or a local Facebook group, that get uniform directly to families who need that exact school's kit. As with any clothing donation, remember to check for and remove name labels first — named school uniform is one of the most common reasons a shop has to refuse a donation.
Check the directory to see which local shops currently take kids' clothing, or ask about uniform specifically — policies vary more here than for everyday clothes.