Can an argument be valid but not sound?

Certainly!  We experience this all the time.  An argument that follows logically from premises to conclusion is valid, even if the premises are false.  Soundness occurs when true premises follow logically to the conclusion.  Building skills is the focus of our School of Logic and one of the skills we seek to develop is the ability to think well.  Aristotelian Logic courses followed by Debate are part of the curriculum for our students in grades 6 through 8.  In these courses, students learn the elements and rules of argumentation.  Rhetoric School students build on that knowledge in competitive debate opportunities through the National Forensics League.