If there were rules in place you have to follow through. Whatever the consequences you set up, you must follow through on them. If you told your child beforehand that if you find them texting to the opposite sex you will take away the phone then that is what you must do. If you never spoke about it then this is the time to sit down and talk about it.
The talk will have to be appropriate to the child’s age. Discuss the rules of their school and the consequences, the home rules. Why you don’t approve, what the problems are. Ask who the other person is, how they know each other, if there are others? Ask to meet that person and speak to both of them and explain why you don’t approve even though you have nothing against the other child and you are sure s/he is a fine person, this is not the right time in their lives that they should be friends. Now is the time to concentrate on their education. Now is the time to establish good friendships with their classmates because they will soon be off to E”Y and they will soon be deciding who they wish to go with. Getting involved with each other even by text can lead to setting up meetings with each other and can ruin their chances for whatever they have planned for the future. They can get kicked out of school, they can lose their friends and so on. Is it worth it for this little bit of fun? They can be playing with fire and they don’t even know it.
Of course they are interested in seeing what its like to talk to each other, after all they are only human and Hashem has made them that way. It is the changes that are taking place in their bodies as they are growing and maturing and again that is Hashem’s way of slowing preparing them for when they will be fully grown and old enough and mature enough to get married. But not yet, they will have to ride the yetzer harah and have patience until the time is right. They will have to reign in their curiosity and interest and hold back, just like they hold back from turning on lights on Shabbos and trained themselves from eating milk after meat, they will have to train themselves to wait until the time is right and they are ready for shiduchim. Then they will be able to satisfy their curiosity date, figure out what’s going on and what type of girl/boy is right for them for their future. And no one will be in control of ruining their future at that time.