Opt for higher fats- but try to make these healthy fats. Things which are low in saturated fat, try vegetable based and cold products as fat becomes unhealthy once its been heated up.
Opt for carbohydrates and more of them- they tend to be easier to eat in bulk and if your having problems trying to gain weight can help give you extra calories.
Try drinking milk, full fat and add some chocolate powder for taste or honey/sugar. One in the morning with breakfast, one after your workout with your post workout snack and one before bed. Aim for a 300ml glass, it may feel hard at first but eventually will feel easier.
As for being a student, I am sure there are multiple other empty calories you can easily consume! But watch for your weight gain, you want lean muscle not fat to grow so keeping your protein intake on top form is also important.
Milk, dairy, eggs, meat, fish, tofu/soya, nuts...etc all have protein in, aim for 2.5g per KG of your body weight. You will need to do the maths, for eg there are around 4-8g of protein in an egg depending on its size, about 35g in the average 125g chicken breast...you need to keep on top of that and work out what you need and when your going to eat it. Preplanning every meal is where its at. Its likely to feel like its impossible to keep up for the first week at least but keep going and you will find it becomes second nature.
It also may be the case that you need an additional meal/snack or add more to what your already eating. Weigh and measure everything so you know how many calories your eating, theres no point in adding in all this extra food if your not sure if its enough. Keep what your eating in a log book so you can notice trends eg when its easier to eat more or harder to eat enough so you can evaluate and better your diet each week.
Funds may be low but bulk buy and shop around- its not rare to go through Uni/College and put on more then a few pounds trust me!