John Bowlby developed attachment theory and proposed that there are four main stages of attachment in infants and children: pre-attachment, attachment-in-the-making, clear-cut attachment, and goal-corrected partnership. He also identified four categories of attachment styles in children - anxious-avoidant, secure, anxious-resistant, and disorganized. Attachment theory suggests that a child's early relationships with caregivers form an internal working model that guides their relationships later in life. The theory can help educators understand students' relationship difficulties and apply interventions accordingly.