Search the internet for "reverse directory". The ones that will give unlisted numbers and cell phone numbers charge a fee, but any published numbers are available.
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You need to check your local phone directory, or do a search for the school on the internet. Phone numbers are often outside a school as part of a notice board.
You can't; due to many people been kidnapped by finding their phones and looking their addresses, most home phones are kept secret. Also white pages have ceased to be published duo to this same...