The Census Bureau defines households in the ACS as including all persons who are “current residents” of the housing unit. This includes everyone who is currently living or staying at a sample address, except for those staying there for only a short period of time (two consecutive months or less). Under this rule, anyone who has been or will be living for two months or less in the sample unit when the unit responded (either by internet, mail, telephone, or personal visit) is not considered a current resident. More details about residence rules as well as a list of exceptions is provided in chapter 6 of the ACS Survey Design and Methodology Report.
Regarding college students, I recommend reviewing Isabel’s detailed response to this forum question: ACS Sampling of Off-Campus College Students. In short, college students who are not living at home at the time of the household interview will not be included in the household roster. They may be sampled separately in their college dormitory as part of the group quarters sample, but no information about their family will be available in this case. College students who are living at home at the time of the interview and will be staying there for at least two months (such as for summer break) will be included in their household roster. The ACS does not sample college dormitories during summer months.