I am not an expert on New Haven but have lived in the area for 8 years. Aspen Glen Apartments were pretty nice. I lived there in a 2 br when I first moved to New Haven area and I liked the sidewalks around the complex which made for a nice safe walk in the evening.
Re: close to Hamden Hall (a private school in Hamden.) I drive by that area frequently as it is close to Hamden's nicest park for children and I saw at least 4 apartment buildings within 3 blocks of Hamden Hall. Some of the names of the complexes are Lakeshore Apartments (unknown contact) , Lakewood Luxury Apartments web site:
www.hamdenapartments.com, and really nice condos complex called Whitneywood. These are all on a busy street called Whitney. There are many more apts just down the road in a nice part of New Haven also walking distance to Hamden Hall. I had a friend who was a doctor rented at the Claremont Apartments and she really liked it:
www.trumbullenterprises.com
I found my latest apartment in West Haven on Craigs list so that is also a good place to look.
Good Luck.