t3.xlarge offers 4 vCPU and 16GB RAM while r6g.large offers 2 vCPU and 16GB RAM ie. r6g instance offers more memory ratio. Also t3x.xlarge is burstable instance which means that the performance is limited to sustain at around 20-30%, anything more than that it will need to consume burst credit.
Usually to estimate whether your current capacity is sufficient, you would need to do a load test. The maximum instance capacity is usually limited to a number of users it can handle concurrently (ex: 100 users concurrent or per second). So although 1 day the school can have max 5000 students, you still need to know how many out of 5000 may access the web app concurrently. It is probably much less unless it is exam period.
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