It’s generally very easy for tech-savvy kids to circumvent many parental control restrictions, including iOS Screen Time restrictions, if they’re not defeating the entire parental control app. The easiest way is for them to simply uninstall the app. Top parental controls, like Qustodio, come with uninstall protection that prevents this. Even if it doesn’t have uninstall protection, a good parental control app will send you a notice that it was uninstalled from your kid’s device, so you can have a discussion with them.
To get around uninstall protections, kids with a little money can simply buy an older, used iPhone without you knowing it. If this is a concern, I recommend getting Bark, which not only monitors devices, it monitors accounts. That way you can receive notifications of any concerning content or interactions your kid has on social media or in chats, regardless of the device they use.