You will need to replace either the charging flex cable, as it houses a microphone, or the Proximity/Front camera flex cable. Depending on whether this issue is present while speakerphone is enabled or not.
There are usually legal consequences that can be involved in these kind of situations. The computers are owned by the school or district and bypassing security measures without administrative permission could result in expulsion as well as legal ramifications as mentioned.
In my experience, in the cable area, its usually because the cables are not tucked in properly and will usually result in pinched/severed cables. Please take care to make sure the cables are moved into the proper position before applying too much pressure. Also please don't use glue, it makes future repairs extremely difficult. Tessa Double Sided red tape works wonders as well.