Computational thinking (CT) has been recognized as an important part of K-12 education in many countries and is often described in term relating to computer science both in concepts and context. However, the unique practice of computer scientists collaborating on problem solving is often overlooked in CT education in high school. We believe that collaboration is a key aspect of modern computing, and propose to integrate collaborative CT activities into existing high school education. The aim is to prepare students better for exploring computational approaches to high school disciplines as they are practiced in the professional world. Our research question is: how can we design tools for supporting students' collaboration while teaching CT in high school? A new computational environment, CoTinker, has been developed. The tool uses a cross-device approach and foster students' verbal communication and collaboration in groups. The tool was tested in a Biology class. Students' computational thinking (CT) was assessed through learning activities and students' communication was analyzed by audio recordings, of students, imbedded in CoTinker and by focus group interviews with students. Preliminary findings showed students being able to collaborate and solve CT tasks in groups independent of teacher or researchers. Results also indicated that students were able to use, navigate, and critically reflect upon the CoTinker environment.