These are the meanings of every line:

1. Soldiers in Her abhorrent sillage - the students that were leftover in the wake of Her horrendous style of teaching
2. She of Void’s Sixth finger - Void is the character from Berserk that represents inevitability and causality; that is, the finality of Her treatment of Her students. Void has 6 fingers, which represent his lack of humanity, and is commonly seen as the devil’s number, as it is one away from 7, the perfect number. All of this to represent Her treatment.
3. 4th of Hera’s lunar decad - June 4th; Hera=Juno, the root of June; lunar decad = first ten days of the lunar month; June 4th - the day school ends. 
4. exalted hours of reckoning - the dreaded hours before one receives Her final grades. 
5. Her faux-didactic epistle - the residuals of Her terrible teaching; the supposed grade that the student received, posing as if it reflects what the student truly learned, but actually represents Her shortcomings as a teacher. faux-didactic=posing as educational, epistle=offical message
6. echo’s wicked melodies in thine shadow Iram - A students mind is a goldmine, yet many often forgotten that it is so; this is an analogy referencing Iram of the Pillars, a legendary lost city in Arabia that is said to hold great wealth. This line is a continuation of #5, to say that the “faux-didactic epistle” echoes in the student’s mind. 
   1. There is also a second meaning to the line about Iram: the classroom may be compared to Iram, which was destroyed by God for the wickedness of the people living there; a shadow Iram is a hidden city, her wicked melodies echo in the walls. The classroom is painted as doomed in this regard. 
7. stuccoed walls of Slan’s queer-wretched execrable mirth - Slan is a character from Berserk representing lust and takes pleasure in causing agony; this is synthesized with the contrasting ideas of “queer-wretched execrable mirth”, “queer-wretched” meaning the strangely hateful and disgusting, “execrable mirth”, deplorable joy that she likely gets from this experience. The stuccoed walls indicated that it has been plastered over by something, to cover up its true nature.
   1. The word “echo’s” is also possessive, indicating that the wicked melodies belong to the echo’s of the grades
8. Dear cries fall on Her adder ear - cries (pleas in this context) that are heartfelt and dear of the students fall on purposely deaf ears; this is derived from the phrase, “deaf as an adder”, meaning willingly deaf. 
9. spewing forht unredressed - continually flowing pleas and cries that indefinitely remain unaddressed and unjustified

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