"Circa" is Latin meaning "around" or "about". This photograph does not have a date inscibed on it, but is believed to be from around 1860. How do we know? The first clue to the date of this photograph are those subjects that can be identified. John Woodbridge Dickinson was principal from 1856 to 1877. James Greenough was his assistant from 1856 to 1871, so this provides a broad time frame. Beyond this, clothing, hairstyles, and even the carpet provide clues. Historical clothing expert Carolann Schmitt provided this analysis:

Necklines:
The necklines on most of the dresses are at the base of the throat, characteristic late 1850s to early 1860s. In late 1863 to early 1864 necklines begin to creep up on the neck, getting higher and narrower.   

Exceptions in this image:

• The young woman in the second row, third from the right, is wearing a dress with a wide open neckline called an “infant bodice”, named for the neckline commonly used on dresses for children and young ladies. It’s appropriate for the apparent age of this female for late 1850s to early 1860s; she’d be a bit out of fashion c.1864 and beyond.    
• The girl in the center of the third row is also wearing a dress with an infant bodice. She’s also wearing a sheer pelerine over top of it to cover her shoulders. A pelerine is an accessory used to fill in an open neckline. It is triangular in shape in the front and back, usually made from net or a very sheer fabric, and often trimmed with rows of ribbon. You can see the three parallel rows of ribbon on the pelerine, and the brooch to hold it in place. Again, this is age appropriate and in style for late 1850s to early 1860s.  
• The girl in the second row on the far right is wearing a dress with a shallow V neckline. This shape, combined with the slightly wider collar, was typical for the very late 1850s.

 
Collars and Neckbows:

• Narrow white detachable collars were commonly worn to protect the neckline of the dress. The size and shape of these collars were popular c.1860-1863.
• Neckbows were also very popular and an alternative to brooches. The size and shape of these bows date c.1856-1862.

 
Sleeves:

• Many of the women are wearing dresses with full open sleeves. A detachable white undersleeve is worn underneath. Full open sleeves were popular from the mid-1850s through the first years of the 1860s. The size and specific shape of these sleeves were popular 1858-1862.
• A few of them are wearing full sleeves gathered into a cuff, popular during the same period.
• I do not see any indication of the very popular coat sleeve that came into fashion c. 1862.

 
Dress fabrics, women’s hairstyles, and jewelry are all characteristic of mid-1850s-mid1860s.
 
Styles for men’s clothing didn't change as rapidly.

• All appear to be wearing frock coats, white shirts, vests and trousers – characteristic attire for gentlemen c.1850-1860s. A closer look at the vests, shirt collars and cravats helps narrow down the date.
• The vests are single-breasted, have shawl collars, with necklines ending above the middle of the chest – characteristic of vests c.1855-1865.
• Shirt collars are lower on the neck than found in the mid-1850s. Most appear to be fold-down, although the gentleman on the right in the front row may be wearing a standing collar. They are not as low as the collars that begin to appear c.1863 and later.
• There are a variety of cravats. The cravats worn by the three gentlemen in the front row are wider, have longer tails, and appear to be tied by hand – styles common in the late 1850s.
• The two men on the right in the fourth row are wearing more up-to-date styles. The man on the right is wearing either an early version of a 4-in-hand or a narrow looped cravat.  The man on his right is wearing a cravat with a smaller bow and shorter ends. The gentleman second from the left in the back row is wearing a fashionable narrow cravat with a small bow and short ends. I would not be surprised to find these cravats are pre-tied.
• The men’s hairstyles are  characteristic very late 1850s-early 1860s. The facial hair on the man second from the left in the front row is mid-1850s.
• The men in the front are wearing low shows or ankle-high boots, all c.1855-1870.

 
The carpet appears to be ingrain – typical for the mid-19th century.