It depends on what you mean by "site certifite", but this answer is for the case of a root certificate authority:
Most schools - at least in the UK - I don't know about other countries, use some form of web filtering. This typically involves a firewall/proxy that intercepts web traffic and inspects the content of the page.
However HTTPs provides end-to-end encryption between the user's computer and the website, as a result when connecting to a HTTPs website all the proxy would be able to see is the destination IP address, but no information about the contents of the web page.
It would not be practical to simply block all HTTPs traffic, so instead to maintain the filtering system requires the school to break the end-to-end encryption model, by terminating the encryption at the firewall/proxy. It then passes on the connection to the user, but it has to be using their own certificate. This setup enables them to read all communication between the user's computer and the website. (A man-in-the middle attack)
HTTPs certificates are used to prevent this occurring maliciously - because the certificate presented was signed by the school's firewall/proxy (rather than a trusted certificate authority, e.g. Verisign) the laptop's software will not trust the connection (you would see a security warning/error message in your browser). However by installing the school's certificate as a trusted root authority, the connection would instead be trusted and your browser would then function as normal.
The consequence of this is that any HTTPS communication from your laptop can be intercepted and read by the school (even though it appears in the browser as being secured).
More generally anyone who had access to the school's certificate and private key, and also had physical access to intercept your laptop's internet traffic, would be able to read your SSL/TLS communications.
Have a look at this vendor for example: https://www.rm.com/products/online-safety-tools/rm-safetynet/ssl-interception
To avoid this, don't install their certificates, and instead use an OpenVPN connection over a port that they are not blocking (try 53, 80, 8080, 443 etc.)