My kids don't lie, they have something to say, they say it,, they want something they ask for it. We've bought them up better than to think that lying will get them what they want. How hard is it for people to believe that kids can be good kids?, its no wonder teens are so misunderstood.
It is not difficult to believe that kids are innately good (some of them anyway), but it is extremely naive of you be under the misconception that your children do not lie to you.
Everyone will lie, stretch the truth, exaggerate or play down the facts to show themselves in a better light, especially children who are in trouble at school or elsewhere.
This is not an attack on your parenting, just a truth that you need to accept. No matter how good you are at teaching them right from wrong they will still lie to you.
OP, This is simple really, you need to calmly go to the school on Friday and ascertain the whole reason behind the exclusion (it will almost certainly be more than simply a scratch issued while arguing over a school book) and then you need to work with the school and your child to root out the underlying reason, be it retaliation for bullying, study pressure or just a peer related problem generally, even parental pressure from the other child's parents on the school will need to be dealt with.
Yourself and those tasked to support you and your child will be able to ensure that this doesn't escalate into something far more serious or do long term damage to your child's social and educational development.
Don't worry, a simple exclusion will not affect your child's schooling, neither will it reflect poorly on his chances after school as long as you deal with the situation correctly now.