Why is it when i'm with my friends i can hold back tears but i cant with my parents?

whenever my parents yell at me, no matter how big or small, i always end up going to my room and letting out the tears. but when a teacher threatens to give me a detention for no reason, i can feel the tears, but i can hold it back. or when my friends fight with me (words) and criticize and call me things (you dont wanna know) i can hold back the tears. i try, really hard to when we are yelling at eachother, whether its parents or even my brother (i wanna murder him) i cant hold back the tears and end up going to my room before anyone sees. btw i'm 12. i dont cry that often unless its really bad or this.