100% agree. This is my POV as a recently retired tradesman who had many customers try to wriggle out of paying over 25 years. Set your stall out from day one with regard to terms of business. There is plenty of help out there for the self employed regarding debt recovery and the correct processes. Trading standards is a good place to start, offering templates etc. So you must must must get your paperwork in order. This means following the correct procedure and wording for estimates, contracts, additional work to estimate, payment profile, etc. In my experience if a sole trader is not prepared to do this then don't bother starting up in business. The cold hard fact is there are many individuals who just don't want to pay the full price. Top tips: with every estimate enclose your Terms and Condition in which it should state clearly your stage payment schedule, advanced payment for materials, penalties for cancellation, definition of an estimate, state clearly that acceptance of the estimate either verbally or in writing is a contract. If a customer calls you and asks for a "quote" ask them if they want a fixed price and explain the difference between and estimate which is an educated guess and a quote which is a guaranteed fixed price and subject to re negotiation for extras! Don't work for nothing! Your liabilities to Mr Smart Arse you did a cheap job for are the same as a proper earner so think carefully. Make sure you are insured, read the small print on the policy. I found that my insurers of 10 years had stuck a new clause into my policy preventing me from working on buildings higher thin 15 meters. When I questioned it thinking they must have thought I had become a roofer, I was told it included internal works due to the risk of extensive damage to floors below. To increase to all buildings would have doubled the premium with that company of bandits. Lastly, pick and choose your customers, many on here will know what I mean without me going into details. Sufficient to say that certain ethnic groups of the population have a very different attitude to paying their way in life. So you become very aware of who to not to even do a price for.
My SOP for work with over £250 materials was to get money for materials and waste removal either before starting or on the day work started. No money and I would walk. Then agree stage payments on completion of certain work, eg bathroom ripped out, fist fix, tile installed then balance on completion. My aim was to leave no more than 10% on the job on the last day. Get the cust to sign a satisfaction note.
In the last year of business we got a card machine so we could eliminate one more excuse for not paying on the day.
Get all of the above right and it works. With your current outstanding debt I would call the school secretary on the phone and make an appointment to visit in person to discuss the consequences of not paying for work and materials. Have that face to face and make it clear that you will pursue the debt legally. Go to
https://www.gov.uk/make-court-claim-for-money/going-to-court and download the forms, fill them out and take them with you to show you mean business.
Good luck