No, US immigration officials know that you must complete immigration at the first US airport you arrive in (i.e. there is no "sterile transit" in the US). That means that they are set up to handle this and should not give you any trouble.

The fact that your layover is 20 hours is irrelevant to the border officials. Being granted entry to the US grants access to all 50 states, DC, and some offshore territories such as Puerto Rico without additional checks or applications. Since no distinction is made between access to Massachusetts versus California, they will not care how long you plan to stay in California as long as your intent to start your studies in Boston on-time is legitimate. This also means that you are free, at any time during your studies, to go to Texas for a rodeo or go to Alaska for the Iditarod as long as you maintain your studies.

Make sure you have evidence (e.g. tickets) of your passage to Boston and your student status in case you are asked.

Have fun at BU!

No, US immigration officials know that you must complete immigration at the first US airport you arrive in (i.e. there is no "sterile transit" in the US). That means that they are set up to handle this and should not give you any trouble.

The fact that your layover is 20 hours is irrelevant to the border officials. Being granted entry to the US grants access to all 50 states, DC, and Puerto Rico without additional interviews or applications. Since no distinction is made between access to Massachusetts versus California, they will not care how long you plan to stay in California as long as your intent to start your studies in Boston on-time is legitimate. This also means that you are free, at any time during your studies, to go to Texas for a rodeo or go to Alaska for the Iditarod as long as you maintain your studies.

Make sure you have evidence (e.g. tickets) of your passage to Boston and your student status in case you are asked.

Have fun at BU!