Mobile phones are important for people, especially for young adults and adolescents. As people tend to form attachments to not only social partners, but inanimate targets as well, mobile devices can become important objects that provide safety and security. This could lead to separation anxiety, also known as "nomophobia". Constant need for mobile use may result in problematic behaviors in schools, cause distraction in class, it is important to explore the students' relationship to devices. Our study compares state anxiety level of high school students on a regular school day and on an experimental "mobile-free day", when participants do not carry their mobile phones during classes. We hypothesized that separation from the mobiles would increase anxiety and decrease class engagement, especially in students with higher mobile attachment scores. The sample consisted of 235 secondary school students. Results of Repeated Measures ANCOVA showed that anxiety levels increased on the mobile-free school day, but class engagement was not affected by the experiment. Linear regression analysis revealed 'Safe Haven' mobile attachment to be a significant predictor of state anxiety on the mobile free school day. Moreover, correlation analysis revealed that mobile use habits linked to social media and instant message services were associated with higher anxiety scores on the mobile-free school day. Our results provide more insights on both use of mobile phones in learning environment and regarding school regulations of students' device use.