setting the tone behavior management-wise

Hi SLPs! I’m a fairly new SLPA that started at 2 schools in the middle of the year and I am still off for a few more weeks for summer. I’m planning to return to the same school and was wondering how to set the tone for students behavior wise? I work with middle schoolers and really only a handful of them give me any trouble and at an elementary school which is sometimes a little tougher. I feel like I have alot of room for improvement and often feel like an imposter at my job. I struggle in general with behavior management because I’m “too nice”, a people pleaser, and have a hard time being firm. Don’t get me wrong, I generally enjoy my job day to day and I realize too, being more confident comes with more time in the field. Besides the first session being rapport building, what are your best methods to set the tone for the school year? Do you have any phrases for redirection that are useful in the elementary/middle school setting? Thanks in advance! :)