What should one pay attention to when translating "I will go to military-school [within the next week]" into Chinese?
我即将去军校 is grammatically correct, but 即将 sounds too formal. Try this:


  我很快就要去军校了。 I soon will go-to  military-school le.


Notice that I used 了 which marks the end of what I am saying. Otherwise a listener may expect that you have something more to say. There are subtle uses of 了 that may take pages to explain. 

If you want a formal form, 我即将去军校了 is perfectly fine. 



我下星期里去军校 is understandable. Within is kind of a formal preposition word in English. In Chinese you may want to say:


  我要在下星期内去军校。 I will at next week within go-to military-school.


在...(以)内 is more idiomatic.



我比及读完去军校 -- 比及 is an archaic word that is no longer in use. study is 学习; It can be 读 sometimes, but the context is missing. Anyway:


  我一旦学习完就去军校。 I once study finish then go-to military-school.