I have moderate wifi signal strength in my apartment from the schools network with the internal antenna in my laptop. I want to set up something with a directial antenna towards the school and rebroadcast it within my apartment as my own network so I can avoid giving Comcast any money. I’d like to set something up independent of the laptop.
You will probably have to get a pfSense router or some other kind of special router (I know consumer routers won't do this with their stock firmware at least) and a wifi card with a directional antenna. Then you may be able to set up your WAN as a wireless interface. I can't confirm this as I've never tried, but it's worth checking into.
Cheapest way would probably be to get a router that supports dd-wrt or openwrt and set it up to use its WiFi in client mode to connect to your school. Get one with removable antennas so you can connect directional ones.
You would then want to configure it with NAT on the wireless interface so your local network shares the wireless ip address and have the firewall block inbound traffic. You could do it without the nat firewall but it would work probably work better if you are only using the schools network as your gateway.
You could do the same thing with a computer like @K4KFH suggested, and it would probably be more reliable, but a router would be cheaper.
If you want to spend some money a better way of doing it would be to get a good directional wireless access point which can be put in to client mode and connect it to the wan port on a router. This will probably give you a better signal.
I bought the amped wireless REC10, its a range extender. However when you connect to it you can set it up to repeat your network or broadcast a new one. I then went into the settings and connected it to the schools network which used TKIP encryption. I then setup my network setting through the extender's options. It kinda worked but because the school's signal was pretty weak. I fixed this by replacing the standard antenna with a cheap tp-link 8dBi one that was about a foot long. I'm now big cable free.