Atypical (WHO grade II) meningiomas are characterized histologically by 
((Backer-Grøndahl T, Moen BH, Torp SH. The histopathological spectrum of human meningiomas. Int J Clin Exp Pathol. 2012;5(3):231-42. Epub 2012 Mar 25. PMID: 22558478; PMCID: PMC3341686.)):

4 to 19 mitoses per ten high-power fields
3 or more of the following 5 histologic features:
necrosis
sheet-like growth
small cell change
increased cellularity
prominent nucleoli
direct invasion of brain parenchyma
It is important to note when reading older literature, that it is only since the WHO 2007 classification, that infiltration into brain parenchyma of an otherwise "benign" grade I tumor was sufficient to designate it a grade II tumor. As such, the incidence of grade II tumors increased to ~30% 
((Backer-Grøndahl T, Moen BH, Torp SH. The histopathological spectrum of human meningiomas. Int J Clin Exp Pathol. 2012;5(3):231-42. Epub 2012 Mar 25. PMID: 22558478; PMCID: PMC3341686.)).