If you have no personal associations with school then the dream symbolises learning and knowledge.
Your associations could be authority or indoctrination or if you go to school or are a teacher, then use this angle to understand the message of the dream.
The below suggestions focus on school symbolising learning and knowledge.
You dreamsense it was a school alludes to learning and knowledge around a life lesson, concept, idea, project, etc.
You dreamsesne a specific school grade alluding to a particular phase or stage you are at with a life lesson, concept, idea, project.
A school you attended could represent that time in your life, maybe something that happened there or something that started at that time, like a pattern or behaviour. I have never dreamt of the past unless it relates to something that is currently affecting me today, so identify how it relates to you now.
Another thing to consider is the school you attended maybe highlighting a life lesson, and the specific school level will allude to the stage you are at. This is a pattern I have with my dreams whereby the childhood home symbolises my Life Path and the childhood suburb which includes the primary, intermediate and college that I attended symbolises my Life Lessons. When my dreams have these settings, I give time to understanding them and pay particular attention to the message.
The level of the school symbolises the position, stage, phase and/or progress you are at with a life lesson, idea, project, trait, etc.
An entry level school symbolises that you are at the beginning stage and a high level school symbolises you being at an elevated stage.
Pre-school or Kindergarten alludes to a beginning stage of learning or discovery or was there some basic information being conveyed to you.
Primary or Elementary for those between the ages of 5-10 alludes to improving the learnt basics, this could be the stage of development on a life lesson, topic, concept, etc.
Intermediate or Middle and Junior High School for those aged between 11-13 alludes to being at the halfway point in a stage of development about what is being learnt.
College or High School for those aged between 14-18 alludes to a higher level of learning and more detail about what is being learnt is either needed or is the current stage.
University alludes to higher learning, this is either the stage of development you are at or maybe a recommendation to help you in an area of your life.
Consider whereabouts in the school you were, the teacher, students, objects, dialogue and actions for more information.
Teacher alludes to something you are learning or could learn.
The dominant role of the teacher (specific subject, a person you know from waking life symbolising your association with them, etc) will add to the message of the dream.
A teacher can also symbolise guidance.
Other students allude to the overall mood and theme and if you interact with them, the interaction will add to the message.
Trying to get your ideas or what you know across to the teacher and/or other students alludes to you wanting to get things across to others just like a teacher.
Sitting at the desk alludes to the action of learning, showing you the need to learn.
Items on your desk allude to what it is that you are learning or could learn.
Stationary alludes to communication.
Books allude to information or a topic.
School hallway alludes to a transitional phase
School hall alludes to an announcement or use what was done in the hall to understand the symbolism.
School kitchen alludes to the preparation of what you are learning.
School field alludes to an external aspect (perspective, perception, etc).
School playground alludes to pacing yourself, having a break or fun along the journey.
School carpark alludes to stopping your movement along your path for a period of time to learn something.
School bus alludes to the method or movement along your path to learn something.