What halachick issues should a frum person be aware of when attending a secular college or university? For the purpose of this question assume that said person comes from a yeshivish background without any experience in the secular world.
Below is an excerpt from a letter of the Lubavitcher Rebbe (written originally in English) that discuss attending college; I hope you find this useful.
From a letter of 1 Adar 5722
"Theoretically a college and its faculty should not try to impose any particular views, much less a way of life, on the students. Actually however, the student cannot help being impressed, on the conscious and subconscious level, by the views, outlook and way of life of his professors. These, as well as the whole atmosphere of a college, are unfortunately, not comparable with the Jewish way of life, and frequently if not always quite contradictory to it. This is so even in colleges which are theological, or having so-called religious studies. Needless to say, the whole atmosphere of college is in violent conflict with the Shulchan Aruch way of life, whereby the Jew is totally committed - in every detail and aspect of his personal daily life - to the Torah and Mitzvoth and the service of G-d, as is written "You shall know Him in all your ways", to which a whole chapter in Shulchan Aruch, Orach Chaim (Ch.231) is devoted": note there.
I want to just add that some things to think about you will be Hearing kfira and unnecessary chochmas chitzoni from professors
Interaction with women
My advice to you is if you must go to college best is if you can do it online if not then try at least to sleep at home Otherwise make sure you have good chaver and a mashbia that you are accountable to tell them what your doing and to give one another chizuk
Halachic problems, I don't think you'll find many of. The question that people are dealing with is primarily a Hashkafic one. That said, considering that it will be a secular environment, you will be interacting with women who are unfamiliar with your way of life. Make sure that you are up to scratch regarding the halachos about yichud and negia. Aside from that, just be prepared for the culture shock, because it is a huge change.
As someone who has been to a prestigious university ( I was married at the time), it is most important to make a kiddush hashem. I feel proud that I made one in many ways. I was the only orthodox Jewish student among hundreds and they looked up to me. If I had to come late in the morning because of davvening in the winter they made the lectures later to accommodate me. I am not sure what unnecessary chochma chitzoni mean. My professors just taught their subject and knew it better than many roshai yeshivot know theirs. Most roshai yeshivot learn the blatt as they go along, not knowing the whole masechta in advance. The professors know their whole subject in advance.
I must mention many great rabbonim like the Aruch L'ner also went to university.
Since I cant make comments I have used the answer.
Another kiddush hashem I am very proud of. One of the very big colored men there was away for a few months. When he came back he told me he was put in prison for fighting. He also told me that there was a "small" Jewish person there for financial reasons, Because of me, he took him under his wing and "looked" after him.
I will only touch on a few of the myriad challenges (I hesitate to label them problems). These depend on the specific school and the accommodations it makes for Jewish students, and the student and what he can deal with. Basically, in every area of life (as halacha is pervasive and touches on all aspects of life) college presents alternatives, many of which fly in the face of halacha (at least according to some opinions -- for many of the "problems" there are opinions which help defuse the situation):
classes/tests on Shabbat or Yom Tov
lack of kosher food (even if just outside of set meal times) and no private cooking facilities
lack of eiruv
lack of Jewish community so no davening with a minyan (no sukkah also)
close interaction with the opposite gender/challenges to tzni'ut
social events which (even if appropriate) are on Shabbat/Y"T
required study of inappropriate materials
if you live with a non-Jew then shabbos presents a living lab of amira l'akum questions
if you live with a non-religious Jew then the dorm room on shabbos is its own challenge
of course there are little things - if you are sitting with your non-Jewish friend in your room, while you eat the pastrami sandwich you went out and bought, you can't leave the room and have the meat unattended
candle lighting is often forbidden because of fire codes
This might benefit from becoming a communal list so people can add to it (or add resources for dealing with each item).
the Rambam writes in hilchot deot ch. 6
It is natural for a man's character and actions to be influenced by his friends and associates and for him to follow the local norms of behavior. Therefore, he should associate with the righteous and be constantly in the company of the wise, so as to learn from their deeds. Conversely, he should keep away from the wicked who walk in darkness, so as not to learn from their deeds.
In university you will be bombarded with all sorts of outlooks which are completely contradictory to torah as well as attractive women flimsily dressed. It is a tremendous danger for a good jewish boy sheltered from this in a yeshiva environment.
If you must go there, make sure you have a "Noah's Ark" community to strengthen you with a seder in learning talmud so that you do not get swept away by the waves of falsehood and abominations in the blessed name of tolerance and of "live and let live".
Women also will be exposed to these outlooks and negative environment as well as promiscuous men looking to seduce them. Here too, perhaps the only thing one can do (if one must go) is to be part of a strong torah community nearby to hopefully no get swept away by the powerful current.
also, the chafetz chaim wrote a book called "Nidchei Yisrael" for those leaving their sheltered communities in pre-wwii eastern europe. Some of the issues there don't apply today but alot of the spiritual dangers he raises apply and even more so today.