If your child is telling lies, you are right to be concerned. Let's look an approach that might just get you through such a situation and then think about the general discipline strategy with him going forward.
Generally, the challenge with this kind of situation is a parent is hearing conflicting opinions and does not know what to believe. I would not tackle this situation but leave it unresolved and tell him so--that you are going to leave it unresolved. This will produce guilt in him and leave the problem to fester in his own soul--that you were not a pushover but also are trying to not discipline him unfairly (according to him, if he is so fearful that he has actually created a delusion in his own mind about the event that he believes to be true). If this is a character flaw it will happen again and again where you WILL see the defiance straight on, and then you can tackle it with a gusto.
Currently, you might say to him,
I was not there to see this situation, but God saw it--so we will keep this unresolved. I don't know if you did or didn't but what I'm concerned about is you 'dancing so near the flame'--testing the boundary so closely." Explain how a moth gets burned when his wings get too near the flame of temptation. Tell him how older high school children and even adults get near the flame, cheat, or steal money, or stop working when their bosses aren't looking. Explain that the reason we tell a lie is ALWAYS because we don't want to suffer the consequences of our action, so we attempt to cover it up. Telling a lie may work with people, but it doesn't work with your soul and if you continue in it, you will eventually become a crooked tree. Every time you lie in the future you will go to bed with a guilty conscience before God. It is DIFFICULT to tell the truth in every situation. Difficult even for adults. Dad and Mom want you to learn to respect instructions of adults for your good. That rule was set for you because the teachers LOVE the children and didn't want them to get hurt. There would be no reason to rebel against a good boundary that provides safety for you. We want you to write an apology to the teachers for disrespecting their authority and for not staying away from the area of the chairs. And we want you to know that we will be watching you very carefully for any future disrespect of either of us or any teacher's instructions, and any lies that always follow such disrespect. You WILL suffer consequences from your parents.
Proceeding forward, I would share with him the story of Ananias and Sapphira and how God was telling that entire church how serious a lie is, by making an example of these two. Then I would also take him to a jail or a prison and tell him this is what eventually happens to people who lie. Then at home I would land hard on arrogant language--even when there are no lies--telling him that pride goes before a fall. I would make him re-run scene after scene where he is arrogant. Make him enter the room again with a better tone of voice, etc. or he will go without a snack or dessert at the next meal, or have his bike taken away for a day, or, or, or....
Administer consequences with absolute calm on your part; do not show any agitation or disappointment. Take the higher ground in your demeanor and consistently land the consequence, again and again. He'll get the message. Since he will always try to squirm out of the situation by lying, go after the tone of voice instead. Hit the issue at the "letting out of waters."
An additional strategy will be to sit next to your son and use a piece of paper to demonstrate how a lie expands, like when we throw a pebble into a calm body of water and see the waves ripple away from the point of impact. Often, an initial lie is followed by anger by the liar and affects more and more people. When someone leaves the holy path of God, he can expect to find himself in a wilderness. And then reinforce this about sin: sin always takes us further than we wanted to go, makes us stay longer than we wanted to say, and pay more than we wanted to pay.
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