Keep it cool. A calm, steady parent wins the day!
When your children tell you something that might be a surprise or even a shock, be cool about it. Show them that you are very interested in what they said and would like to learn more. You may be really in shock, especially if it's of a big nature. Children will talk more to us about their issues if they think parents are calm and will listen. If they see a reaction, they are more inclined to shut down or not tell us at all.
Keep chatting about what is on their mind. They look for our reaction throughout the conversation.
A friend told me that her son informed her that when he was younger and moved to a secondary school he was offered drugs on his very first day. The horrified mother said that had she known, she would have been up to that school and sorted it out very quickly. The son, (now twenty one years of age) said "and that mum is why I never told you". I suspect that if her son felt his parents would work through the issue calmly, he may have told them.
Our children will talk to us about serious matters if they know we listen with understanding. We are better equipped then to show empathy and work through the problem together. So be cool when chatting about matters that can be serious. This way you respect what they have to say without making early judgements.
When children talked to me about matters concerning them in the school setting, I would usually begin by saying, "It sounds like you have a problem. Thanks for talking to me". This sets the scene for a calm discussion.