Philippians 2:14-16
Oh my dear children, how important it is to have control of your tongue. The book of James speaks greatly about it and its dangers. I encourage you to read that book and take it to heart. I encourage you to listen to His holy Word and obey it... it will be so good for you.
Learning to control your tongue in many areas, will serve you well. I wish I learnt that as a kid instead of having to struggle with that as an adult, so be wise and take that habit now. It is so easy to complain, to criticize, to speak down on others when we know better... but I warn you, that habit is hard to break. You are to speak to the least important person in your eyes as if they were the most important... always showing respect and honor.
Remember that one day, they could come to the church and meet you... and the way you talked to them before will have a great deal of importance in their thoughts about the church. If they remember you always being very kind and respectful, they will have a high esteem for the church and you will not be blamed to bring shame to the faith you profess. But if you were rude, proud, impatient, un-thankful, ... you are being a poor example of the gospel and what Jesus has done for you.
So do not complain when you are asked to do something, when you are at a place and you are not served as you think you deserve, or at school with your teachers... but in all give thanks to the Lord. and know, that all those opportunities, are there for you to show Jesus Christ to others.
Be kind and loving to each other, always saying the truth in love. And you may be able, to boast in the Lord and His work in your life... and that all that mortification of sin was not in vane, but for Him who saved you.