Greetings,
May I suggest a few things?
In order to plant a garden you have to sow seeds. The question could be asked, "What are you planning to have in the garden?"
Ultimately we plant in order to have an outcome: vegetables, flowers, herbs, etc. So when planting we expect a result but we are willing to wait until the due time when the planting brings forth that which we desired when we planted.
Of course, any old seed won't do and likewise any old garden bed won't do, either.
So, for you, your desire to sow must be first.
This is something between you and the Lord. At the beginning of each day, according to your desire, pray and speak to the Lord, asking that opportunity will open for you to plant His seed and to water that which has already been planted, thanking Him for His faithfulness and the Love He has put in you - which is where the desire comes from.
A good way to be ready is to daily read some or all of one of the 31 Proverbs, one for each day of each month.
Thank and trust the Lord that He will open an opportunity for you to say something to another that day.
It doesn't have to be big and long but simply a word spoken at the right time as you talk with someone. A conversation often has plenty of places to add a comment and your comment can fit well and be welcomed. Again, you don't need to push or force your opinion or preach to someone but faithfully plant a seed. Don't seek praise and don't come across as being the clever wise one but know that the Lord can provide opportunities for you to say something fitting and will be with you, always.
The Proverbs are full of wisdom and wisdom is best in small doses with few words as you will realize as you read them.
Rejoice in the LORD.
Remember, but don't restrict yourself or the Lord, that at the age that your fellow students are, there are so many variables happening in their lives and while we might want to be able to explain everything in one go, concentration and interest may often be limited and each will be weighing up everything almost faster than it happens and while you may see no evidence that what you said stayed in or was even heard, if you are faithfully and sincerely wanting to share the Lord with them, He will not only prepare them but also watch over the seeds you plant in His garden.
Trust in the Lord.
I understand how difficult it must be for you or anyone your age to say anything about the Lord Jesus Christ in a public school and I encourage you to press on and fear not but at the same time, be wise, seek wisdom and wait upon the LORD. He will guide you, lead you, show you, go before you and surround you and love you at all times.
Listen to Him, He speaks, for you can not expect another to listen to you more than you are willing to listen to the Lord.
Jesus is Lord and King whether others around you know it or not. You know it for you know Him.
Diligently seek Him and wait upon Him, He loves you.
Bless you ....><>
