Hi R users

I want to use the lme package for a multilevel analysis on the following example:
> math<-c(2, 3,2, 5, 6 ,7 , 7)
> sex<-c(1, 2, 1, 2, 2, 2, 1)
> school_A<-c(1,1,1,2,2,2,2)
> school_B<-c(10,10,10,20,20,20,20)
> mydata<-data.frame(math, sex, school_A, school_B)
> mydata
 
School_A and school_B are two different school characteristics, math is an individual score for each subject and sex is also an individual characteristic. 
Now i want to specify in R the following equations:

math=a +b *sex +error1
a= aa+ bb*school_A+ cc*school_B+ error2
b= aaa+ bbb*school_A+ ccc*school_B+ error3

but i don't know how. I searched nabble but could not find anything.

Thank you and have a great day ahead 
 

       
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